OCR Text |
Show 156 CRUSTACEA. such as the Calymenes, appear to approximate, as well as to the Chitones, inasmuch as by contracting they could also be. come spherical. Since the publication of M. Brongniart's work, some naturalists have rejected his opinions and adopted mine, either wholly or in part; others still hesitate. Be this as it may, these animals appcat· to have been annihilated by some ancient revolution of our planet. 'D1e Tr·ilouites, one heteromorphous genus excepted, that of Agnostns, have, like the Limuli, a large anterior segment in the form of an almost semicircular or lunated shield, fol· lowed by from about twelve to twenty-two segments(J ), all transversal except the last, and divided by two longitudinal sulci into three ranges of parts or lobes, whence their name of Trilobites(~). Some naturalists ca11 them Entomostracites. head of that class, and which leaves between it and the latter Crustacea a conai. derable hiatus. The Calymenes, with respect to this contractility, evidently ap· proach these latter Insects, the Typhes and Sph;eromre; but it does not appeat that the posterior extremity of their body is provided with lateral natatory appen. dages, a negative character which would remove them from the Spha:roma:, but approximate them to Armadillo, anrl particularly to Tylos, where the superior part of the t~oracic segments is divided into three. The study of a well pre· ser~ed spec1men. has convinced me that, like the Limuli, they had eyes placed agamst two prommences, anrl that the cornea was granulous or with facets. The non.existence of the superior antenna: also indicates a new affinity between these same Trilobites and the Limuli. (1) The body of various Trilobites, and particularly of the Asaphi seems to consist, exclusive oft~1e shield, of twelve segments, well separated on' the side&, and of another formmg the post-abdomen, or a triangular or semi-1 t 'I d . . . . unar u, whose lVlSions are superfic1al aud do not cut its edges. In the Paradoxides, on the contrary, the lateral lobes terminate by well marked acute prolongations, and twenty.two of them can be distinctly counted. A species of Trilobite men· tioned by Count Rasoumowski iq hi» memoir on fossils, Ann. des Sc. Nat 'June 1826, pl. xxviii, ii, which he presumes should constitute a new genus, is,· in tb~ ~espect, ve~y remarkable. Its lateral lobes form very long thongs or slips taper· ~ng t.o a pomt. !he feet of the pupa: of the Culices are elongated, flattened, marbculated lammre terminated by threads and fold d th 'd Th · a rudimental state ai d . e o~ . .e SI es. . ey ~re m . . . ' 1 may be analogous to the lateral diVISions of tillS spec1ea of Trilob1te, allied to the Paradoxides, (2) The S~uillre, and var~ous Amphipodous and Isopodous Crustacea have also seve~ of their segments tl·tsected by two impressed and longitudinal lines; but these lmes are ne:Lrcr to the edges and do not form deep sulci. P ...ECILOPODA. 157 AGNOsTus, Brongn. ·,. The only genus where the body is semicircular or reniform. In all the other genera it is oval or elliptical, and exhibits the general characters above mentioned. C.ALYMENE, Brongn. The Calymenes are distinguished from all otlier q'rilobites, by the faculty of contr·acting their body into a ball, and in the same manner as Sphreroma, Armadillo, and Glomeris, that is, by approximating the two inferior extremities of the body. The shield, as broad as it is long, or broader, is furnished, as in the Asaphi and Ogygire, with two oculiform prominences. The segments do not project beyond the sides of the body, and are united throughout; the body is terminated posteriorly by a sort of triangular and elongated tail. In AsAPHus, Brongn. The oculiform tubercles seem to exhibit a sort of eye-lid, or are granulous; the species of tail which terminates the body posteriorly, is less elongated than in Calymene, and is either nearly semicircular, or in the form of a short triangle( 1 ). In the 0GYGI~, Brongn. The shield is longer than it is broad; its posterior angles are extended into a kind of spine. The oculiform tubercles exhibit neither eyelid nor granulations. The body is elliptical. PARADOXIDEs, Brongn. The eye-like tubercles cease to exist, or are not apparent in this genus. The segments, or at least most of them, project beyond the sides of the body, and are free at their lateral extremity. Such are the characters of the five genera established by M. AlexA ander Brongniart, which may be arranged in three principal groups: the Renifonnes-AGNOsTus; the Contractiles-CALYMENE; and the Extensi-AsAPHus, 0GYGIA and PARADOXIDES. (1) In the.Osaphu" Brongniarti, described and figured by M. E. Deslongchamps, the posterior angles of the shield, instea.d of being directed backwards as in the other species, are recurved. |