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 Spliaeroma, 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 Ogygiae, 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 ; 
i\-\e Reni formes — Agnostus; the Contractiles — Calymene; and the 
Exlensi — 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 jiart 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 Brongniarli, 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. 
