OCR Text |
Show 40 these parts are contiguous inferiorly ~long the. int~rnal e~ge, .and form an elongated triangle, the extremity of wh1ch 1s received mto two upper cells of the buccal cavity. The tail, which is ample and suborbicular in the females, usually consists of from four to five segments, but never seven. Doctor Leach( 1) has separated this genus of Fabricius into seve-ral genera, which, however, we will consider as simple divisions. Those species which have a transversal shell, with the middle oi its sides greatly prolonged or dilated, so as to resemble a cone or cylinder, form his genus Ixa(2). Those which have a rhomboidal shell with seven conical points, resembling spines on each side, compose that of !phis. If the shell still has the same rhomboidal figure, but merely presents angles or sinuses on the sides, it becomes his genus Nursia. If these lateral edges are smooth, we have his Ebalia. The Leucosire with an ovoid or nearly globular shell, and otherwise distinguished from several of the preceding by the claws being always longer than the pody and thicker than the other feet, and by the tarsi being sensibly striate, may be divided thus: In some the front projects, or at least is not surpassed by the superior extremity of the buccal cavity. The outer branch of the external foot-jaws is elongated, and almost linear. Here the claws are slender, the hands cylindrical, and the fingers long. Sometimes the shell is nearly globular, and either very spiny, as in the genus .flrcania, or smooth as in ILIA. At others, the shell is suborbicular and depressed, as in the genus Persep!tona, or ovoid as in Myra. There the claws are thick, with ovoid hands and short fingers. They constitute the true Leucosi::e of that naturalist. In the others, the superior extremity of the buccal cavity outreaches the front. The outer branch of the external foot-jaws is short, and arcuated; the shell rounded and depressed. This last division comprises his genus Phylira. Other considerations, founded on the proportions of the feet and the form of the external foot-jaws, strengthen these characters. The Leucosie noyau; llia nucleus, Leach; Cancer nucleus, Lin., Herbst., XI, 14, is common in the Mediterranean; its shell is globular, granulated on the sides and posteriorly; the front is notched; two teeth on the posterior margin, and two others (1) Leach Zool. Misc. III; Desmar., Consid. (2) Leucoaia cylindrus, Fabr., Herbst., II, 29--31. 41 widely separated on each lateral margin; the po~tcl'ior largest and spiniform, and situ!lted, above the origin of the posterior feet. The sea coast of the western departments of France produces some other species which belong to the genus Ebalia, Leach( 1 ). All the remaining ones are ft·om India and Amet·ica. Some fossil Leucosire are found in the East Indies. Three species have been described by M. Desmarest, two of which, according to him, are true Leucosire, Leach, and which are now living in the same countries, and peculiar to them. Our fifth section, that of the TRIGONA, is composed of those species whose shell is usually triangular or subovoid, narrowed before into a point or kind of beak, generally uneven and rough, with lateral eyes. The interval comprised between the antennce and the buccal cavity, is always nearly square, as long, or almost as long as broad. The claws, at least those of the males, are always large and elongated. The following feet are very long in a great number, and sometimes the two last even differ in form from the preceding ones. The third joint of the external foot-jaws is always nearly square or hexagonal, in those at least whose feet are of the ordinary length. The apparent number of the caudal segments varies. In both sexes of several it is seven; in others, however, the males at least, it is less. Several of these Crustacea are designated by the vulgar appellation of .flraignees de mer or Sea-spiders. Although the species of this tribe are very numerous, but two have as yet been discovered except in a fossil state, one of which at least-Maia squinaclo-exists at the present day in a living state, and in the same localities(2). A first division will comprehend those whose second and following feet are similar, and which diminish progressively in size. From the latter we will form a fit·st group of all those where the tail, either in both sexes, or in the females alone, is composed of seven segments. The third joint of the external foot-jaws is almost always square, and truncated or notched at the superior internal angle. Very large claws, particularly so when compared with the other feet, which are extremely short, directed horizontally and perpendicularly to the axis of the body as far as the carpus or joint immediately preceding the hand, then reflected anteriorly on them- (1) Malac. Brit., xxv. (2) See Desmar., Hist. Nat. des Cl'ust. .Foss VoL. Ili.-F |