BKACHIUI'OUA. 11 



and nearly entirely cartilaginous. The family consists only of a 

 single genus, Linyula. 



Order V. Rudistes. 



This Order has been placed by modem authors with the Bra- 

 chiopoda : the proofs of its belonging to this family are not very 

 evident ; but as there is no other to which they a})pear to be more 

 nearly allied, they may as well l)e retained in tliis position. 



Lamarck, Cuvier, Ferussac, and some other authors have re- 

 garded some of the genera as belonging to Cephalopoda, and 

 others as Bivalves (Conchifera). Ueshayes regards them as more 

 nearly allied to Chama, the character of the family having been 

 lost by the destruction of the inner coat of the shell during the 

 fossilization of the specimen. 



M. d'Orl)igny has properly united them into one group under 

 the name of Irregular Brachiopods or Rudistes, but he includes 

 with them the genus Crania, which is a true Brachiopod. 



They form three very distinct families. 



Fam. 1. RadiolitidcB. 



The lower valves more or less elongate-conical, fixed ; the 

 upper valve conical or spiral, free ; the texture of the lower valve 

 cellular or fibrous. 



The Radiolites has the upper valve flat or conical and cap-like. 

 The Caprina, D'Orb., has a spiral and produced upper valve. 

 The first of these genera has had many names applied to it, but 

 that given by Lamarck has the priority. It has been called 

 Sphcerulites, Ostracites and Acardo, and the cast of the interior 

 canty has been considered as a genus, under the name of Biros- 

 tris and lodamia. 



Fam. 2. HippuritidcB. 



The lower valve is elongate, tapering sub-cylindrical, of a solid 

 laminated texture ; the upper valve is nearly flat, and pierced 

 with peculiar pores radiating to the circumference with branches 

 diverging to the upper surface. 



This family contains only a single genus, Hippurites, Lamarck, 

 which has also had many other names applied to it, as Cornu- 

 copia, Orthoceratites, Batolites (or Batholites), Raphanister, and 

 Bitubulites. 



Fam. 3. Caprotinidee. 



The lower or fixed valve is conical and spirally twisted, and 

 marked internally with prominent ridges or transverse septa ; the 

 dorsal or free valve is oblique or spiral. They difi'er from Ca- 

 prina in the valves not being of a cellular or fibrous textui'e. 



This family contains two genera : — 



1. Caprotina, D'Orb., which has the cavity of the shell merely 

 marked with internal ridges. 



