274 CRUSTACEA. 



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. 



CALYMENE, Brongn. 



The Calymenes are distinguished from all other Trilobites, by the 

 faculty of contracting their body into a ball, and in the same manner 

 as Sphaeroma, 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 Ogygise, with 

 two oculiform prominences. The segments do not project beyond 

 the sides of the body, and are united throughout ; the body is ter- 

 minated 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 *. In the 



OGYGIA, Brongn. 



The shield is longer than it is broad ; its posterior angles are ex- 

 tended 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. Alex- 

 ander Brongniart, which may be arranged in three principal groups ; 

 the Reniformes AGNOSTUS ; the Contractiles CALYMENE ; and the 

 Extensi ASAPHUS, OGYGIA, and PARADOXIDES. 



For a description of the species and their localities, we refer the 

 reader to the excellent work of this celebrated naturalist, who in his 

 labours upon the fossil Crustacea, properly so called, or universally 

 admitted as such, has availed himself of the talents of one of his 

 most distinguished pupils, M. Desmarest, frequently referred to by 

 us, not only with respect to this particular part of the science, but 

 in relation to his work on the living Crustacea. Different naturalists 

 have proposed various generic sections of these fossils ; but being 

 restricted to general considerations, I have adopted those presented 

 to us by the best work hitherto produced on the subject. 



* In the Asaphus Brongniarti, described and figured by M. E. Deslongchamps, the 

 posterior angles of the shield, instead of being directed backwards as in the other 

 species, are recurved. 



