PiECILOPODA. 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 cha- 

 racters 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 Sphseroma, 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 Ogygije, with 

 two oculiform prominences. The segments do not project beyond 

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

 nated 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(l). In the 



Ogygia, Brongn. 



The shield is longer than it is broadj its posterior angles are extended 

 into a kind of spine. The oculiform tubercles exhibit neither eye- 

 lid 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, 



(1) In the ^sapkus 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. 



