164 



MOLLUSC A. 



and drifts away on the surface of the sea, the contents of the ova 

 to be eventually warmed into a free life. 



According to Carus there are 12,000 species in this order, 

 which he distributes into upwards of fifty families. The order 

 has also been subjected to a somewhat cumbrous set of divisions, 

 the secondary ones dependent on characters derived from the 

 radula, except in Siphonostomata and Holostomata, where the 

 character is taken from the form of the mouth of the shell, pro- 

 duced and corresponding to the siphon of the mantle in the 

 former, while it is rounded and entire in the latter. The table 

 below will perhaps be sufficient to show these divisions, and the 

 families, here admitted, that are placed under them. Claus, how- 

 ever, makes only two sections Cyclobranchia and Ctenobran- 

 chia, the latter comprising Scutibranchia, Pectinibranchia, and 

 Neurobranchia, this last being included in his " group " Tsenio- 

 glossa, while Scutibranchia is conterminate with the group 

 " Ehipidoglossa " (=Aspidobranchia). 



CYCLOBRANCHIA. 



Polyplacophora 

 Docoghssa 



SCUTIBRANCHIATA ... 



PECTINIBRANCHIATA. 



Siphonostomata. 



Tcenioglossa ... 



Toxoglossa 



Khaokiglossa. . . 

 Holostomata. 



Ptenoglossa ... 



T&nioglossa ... 



Chitonidge. 

 Patellidfe. 



Fissurellidae, Haliotidae, Trochidse, Pleuro- 

 tomariidaa, Neritidse. 



Strombidge, Doliidae, Cyprasidse. 



Conidae, Terebridae. 



Muricidae, Buccinidae, Volutidae. 



lanthinidae, Scalariidas, Solariidae. 

 Cerithiida3, Melaniidae, Pyraniidellidas, 



Turritellidae, Vermetidas, Xenophoridaa, 



Naticidaa, Calyptraeidas, Littorinidae, Pa- 



ludinidaa. 

 NEUROBRANCHIATA (=Pulmonata operculata). Helicinida?, Cy- 



clostomidae, CyclophoridaB, Aciculidas. 



