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 appear to have been annihilated by 

 some ancient revolution of our planet. 



The Trilobites, one heteromorphous genus excepted, that 

 of Agnostus, 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(l), all 

 transversal except the last, and divided by two longitudinal 

 sulci into three ranges of parts or lobes, whence their name 

 of Trilobites(2). Some naturalists call them Entomostracites. 



head of that class, and which leaves between it and the latter Crustacea a consi- 

 derable hiatus. The Calymenes, with respect to this contractility, evidently ap- 

 proach these latter Insects, the Typhes and Sphseromx; but it does not appear 

 that the posterior extremity of their body is provided with lateral natatory appen- 

 dages, a negative character which would remove them from the Sphxromx, but 

 approximate them to Armadillo, and particularly to Tylos, where the superior 

 part of the thoracic segments is divided into three. The study of a well pre- 

 served specimen has convinced me that, like the Limuli, they had eyes placed 

 against two prominences, and 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 of the shield, of twelve segments, well separated on the sides, 

 and of another forming the post-abdomen, or a triangular or semi-lunar tail, 

 whose divisions are superficial and 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 in his memoir on fossils, Ann. des Sc. Nat. June, 

 1826, pi. xxviii, ii, which he presumes should constitute a new genus, is, in this 

 respect, very remarkable. Its lateral lobes form very long thongs or slips taper- 

 ing to a point. The feet of the pups of the Culices are elongated, flattened, 

 inarticulated laminae terminated by threads and folded on the sides. They are in 

 a rudimental state, and may be analogous to the lateral divisions of this species of 

 Trilobite, allied to the Paradoxides. 



(2) The Squillx, and various Amphipodous and Isopodous Crustacea have also 

 several of their segments trisected by two impressed and longitudinal lines; but 

 these lines are nearer to the edges and do not form deep sulci. 



