thorax, connate, rounded, not convex, with nine dorsal furrows, and no marginal one; the apex is slightly sinuate.

The surface is rough, without lustre, and moderately coarsely punctured.

This very singular insect is found in northern California, probably near the Sierra Nevada; of its habits nothing is known.

FAM. IV.-DYTISCIDAE.

Mentum deeply emarginate, broadly toothed in the middle; lobes somewhat acute; sides rounded, converging in front; gular suture distinct; ligula large, quadrate, corneous.

Maxilla with the outer lobe biarticulate, the inner curved, acute at the apex, ciliate internally.

Eyes rounded, never emarginate.

Antennæ inserted under the front, behind the base of the mandibles, glabrous, polished, usually filiform, 11-jointed (rarely 10-jointed).

Prothorax with the epimera and episterna distinct; prosternum compressed, produced behind and fitting into a cleft or emargination of the metasternum; anterior coxæ protected behind by the mesosternum, subconical.

Metasternum short, pointed behind, but very closely connate with the posterior coxæ.

Posterior coxæ very large, usually oblique, contiguous at the inner margin, reaching the side of the body, entirely cutting off the ventral segments from the metathorax; internally with a small dilatation for the insertion of the legs, or a broad plate (Haliplini) extending over their whole surface, and also covering the greater part of the abdomen.

Abdomen with six ventral segments, the three anterior ones connate, the sixth rounded at tip, usually permitting the seventh internal but corneous one to be slightly visible.

Legs ciliate with long hairs, posterior usually compressed, elongated, formed for swimming; tarsi 5-jointed, the fourth joint of the anterior and middle tarsi sometimes obsolete.

In this family are contained aquatic carnivorous insects, having, as will be seen by the above characters, a close relationship to Carabidæ, and in fact only differing by the form of the posterior coxæ, and the natatorial legs. The particular portion of the Carabide which approaches most nearly these insects is found in the

first sub-family. In common with that series, the Dytiscidæ have the side pieces of the mesosternum diagonally divided, with the epimera reaching the coxæ; the side pieces of the metasternum are narrow, without visible epimera. The middle portion of the mesosternum is entirely covered; on separating the prothorax, it is found to be deeply sulcate for the reception of the prolongation of the prosternum, which thus firmly unites the different parts of the body, so as to give the stability necessary for rapid motion through the water.

The body is rounded, sometimes elongated, usually elliptical, rarely very convex. The species are more numerous in the northern parts of the country; they are all winged, and the elytra are always marked with three discoidal, irregular rows of punctures; rarely the elytra are sulcate (Copelatus, females of certain Dytiscus), frequently punctured, sometimes sculptured with transverse lines (certain Colymbetes).

The genera represented in our country form two sub-families, so distinct that they might with much propriety be considered separate families.

Sub-Family I.-HALIPLIDAE.

Antennæ 10-jointed; abdomen covered with large plates of the posterior coxa, which are small and transverse; metasternum almost truncate behind, scarcely emarginate in front; legs scarcely natatorial.

This series consists of a single group, containing but two genera, Haliplus and Cnemidotus, of small size; the body is very convex, somewhat acute before and behind, yellow shining, spotted with black or gray. The elytra are covered with rows of punctures; in Haliplus there are nine and a marginal series, in Cnemidotus there are eleven or twelve. The scutellum is not visible. The insects of this sub-family swim but feebly, and with little activity. Palpi subulate.

Palpi filiform.

HALIPLUS. CNEMIDOTUS.

Sub-Family II.-DYTISCIDAE (genuini).

Antennæ 11-jointed; posterior coxe without plates, very large (except in the European genus Pelobius); legs very natatorial (except in the same genus).

Four tribes are represented in our fauna, which may be thus

distinguished:

Anterior and middle tarsi with the third joint bilobed, the fourth scarcely visible.

Tarsi with five distinct joints;

Prosternum dilated behind, truncate.

I. HYDROPORINI.

II. NOTERINI.

This group contains only small species, having the usual form of this family, but rarely very broad, and sometimes nearly globose. But two genera are found with us: Hydroporus, of varied form, having no visible scutellum; Celina, with elongate body, obtuse before, pointed behind, with a distinct scutellum.

More than eighty species of the former genus are known to me,* and species are found in every part of the country.

Tribe II.-NOTERINI.

The form of the prosternum in Noterus and the three genera below mentioned is so remarkably different from that seen in the rest of the members of this family, that they seem to be naturally placed in a special tribe; the prosternum is elevated, very much dilated behind the anterior coxæ, and truncate; it reaches the metasternum, which is also widely truncate in front.

The form of body in these genera is likewise peculiar. In Colpius Lec. it is almost globose, as in certain Hydroporus; in the others it is oval, very convex, acutely pointed behind; the labial palpi are dilated in our genera, sometimes very much so; the antennæ have the middle joints more or less dilated. The scutellum is not visible. No species has yet occurred in the Pacific district. Our three genera may be thus distinguished:

Prosternum deeply concave; body globose; palpi emarginate at tip.

Colpius contains but a single species, found in Louisiana.

*For a synopsis of them, vide Proceedings of the Academy of Natural Sciences of Philadelphia, vii. 290.

Tribe III.-COLYMBETINI.

The only character by which this tribe is distinguished from the next is the form of the dilated anterior tarsi of the males. The dilated joints are here oblong and of equal width, covered beneath with cups of equal or nearly equal size. The prosternum, as in it, is narrow, compressed, and pointed behind. The form of body is oval, not very convex. The species are small or moderate in size.

Our genera may be thus tabulated:

Scutellum distinct.

Scutellum invisible.

2 Last joint of palpi truncate or rounded.

Last joint of palpi emarginate.

3 Prosternum not sulcate.

Prosternum with a deep groove.

4 Claws of posterior tarsi unequal, the outer one fixed.

Claws of posterior tarsi equal, movable.

5 Elytra striate.

Elytra smooth.

6 Thorax narrowed at the base.

Thorax as wide at the base as at the middle, or wider.

2

LACCOPHILUS.

3

COPTOTOMUS.

4

MATUS.

COLYMBETES.

5

COPELATUS.

6

ANISOMERA.

AGABUS.

I have included in Colymbetes the genus Ilybius Er., and entirely agree with Lacordaire that the difference is too slight to enable them to be retained as distinct.

I am also inclined to believe that Copelatus and Anisomera should be united with Agabus; the difference of the first is merely in the striate elytra; that of the second in the thorax, being narrowed from the middle to the base.

Tribe IV.-DYTISCINI.

This tribe contains in the genera Cybister and Dytiscus all the large species, and in the other genera species of moderate size. As before mentioned, it only differs from the last group in having the joints 1-4 of the anterior tarsi of the male of unequal width, so as to form a round disk, furnished beneath with cups of very unequal size. The scutellum is always visible. The body is oval not very convex, usually broader behinde middle, rarely elliptical; the elytra of females of certain Dytiscus and Acilius are sulcate. All the genera of the tribe are represented in our fauna, and may be distinguished thus:—

3 Last joint of palpi not elongated.

Last joint of palpi elongated.

Claws unequal, the upper one fixed (last joint of palpi not elongated); 4

4 Body ovate, intermediate tarsi of male not dilated. Body elliptical, intermediate tarsi of male dilated.

DYTISCUS. EUNECTES.

ACILIUS.

HYDATICUS.

FAM. V.-GYRINIDAE.

Mentum deeply emarginate; lateral lobes rounded; gular suture distinct.

Ligula large, quadrate, corneous, filling the emargination of the mentum.

Maxilla with the outer lobe usually wanting, sometimes slender, not articulated, the inner curved, ciliate internally, acute at tip.

Eyes divided by the sides of the head, upper and lower parts both rounded.

Antennæ inserted under the sides of the front, behind the base of the mandibles, short, thick, third joint auriculate, subsequent ones indistinct, last joint elongate.

Prothorax with the prosternum short and carinated, epis terna and epimera distinct, the latter large.

Mesosternum very large, rhomboidal, posterior angle emarginate for the reception of the point of metasternum; episterna and epimera entirely connate, attaining the middle coxæ. Metasternum very short, pointed before and behind; episterna very large; epimera not visible.

Coxæ, anterior, small, globular; middle, flat, oblique, almost reaching to the posterior coxæ behind; the latter are large, truncate anteriorly, contiguous at their inner margin, extending to the margin of the body, and thus separating entirely the ventral segments from the metasternum; they are dilated internally, and broadly excavated behind for the motion of the hind legs.

Abdomen 7-jointed, the three anterior segments connate, the first almost obsolete; the seventh longer than the sixth, rounded at tip.

Anterior legs very long, received in oblique grooves of the pro- and mesosternal segments; tibiæ slender, with one terminal spur; tarsi 5-jointed, of the male sometimes dilated.