INTRODUCTION. 49 



They compose the genera Gryphaea, Ostreea, Vulsella, 

 Placuna, Anomia. 



6. Rudisf.es. (plates 193 to 200). This family is composed 



of a particular association of shells, which appear on one 

 side to be connected with the Ostracea ; and on the 

 other to approach the Brachiopoda. They differ from 

 Ostracea in having no hinge or ligament, and only re- 

 semble them in their irregularity and foliaceous struc- 

 ture. The following six genera are placed by Lamarck 

 in this family: — Sphaerulites, Radiolites, Calceola, 

 Birostrites, Discina, Crania. Of these, Calceola, Dis- 

 cina, and Crania are shewn to belong to the Brachio- 

 poda. 



7. Brachiopoda (plates, fig. 201 to 219). The shells of 



this family are inequivalve, equilateral, and attached 

 to marine bodies by a tendon passing through one of the 

 valves. The animals have, near their mouth, two elon- 

 gated, ciliated arms, which are spirally rolled when at 

 rest. The following genera are enumerated by Lamarck, 

 Orbicula, Terebratula, Lingula. 



MOLLUSCA. 



Lamarck applies, or rather restricts, this name to those in- 

 vertebrated animals, which while they are inarticulate in all 

 their parts, have the head sufficiently advanced at the anterior 

 part of the body to be distinguished ; which is not the case 

 with the Conchifera. All the shells are univalve, and are 

 divided into six orders, namely, the Pteropoda, which have 

 wing-shaped natatory organs or fins, and have light, thin 

 transparent, nearly symmetrical shells ; the Gasteropoda, 

 with the foot not distinguishable from the rest of the body, 



