134 



MANtJAL OF THE MOLLUSCA. 



nay, often seemingly producing results opposed to a priori 

 probability, and at variance with the suitableness of con- 

 ditions ?* 



Tbe answer is, that each fauna bears, above all things, the 

 impress of the age to which it belongs. Each has undergone a 

 series of vicissitudes up to the time when its barriers became 

 fixed, and after its isolation it has known no further change, 

 but decline. 



The number of living and fossil species of each genus of 

 moUusca will be stated in the following pages, so far as they can 

 be ascertained. With some modifications, these numbers give the 

 following totals, by which the relative numerical development 

 of the orders and families will be seen. 



Eecent. Fossil. 

 CephaIiOPODA. Dibranchiata. 



Ai'gonautidse 4 2 



Octopodidae 63 — 



Teuthidse 104 31 



Belemnitidse — 140 



Sepiadse 30 16 



Spinilidae 3 — 



204 189 

 Tetrahranchiata. 



Nautilidae 6) „ 



Orthoceratidse . , , — ) 



Ammonitidse — 1600 



6 2193 

 Gasteropoda. Pro.sobranchiata> 



Strombidset 87 393 



Muricidse 993 703 



Buccinidae .,. 1,144 352 



Conidse 856 462 



Volutidse 686 210 



Cypraeidae 227 97 



Naticidse 263 340 



Pyramidellidae 216 394 



Cerithiadse 192 610 



Melaniadee 424 50 



TuiTitellidset 329 290 



Littorinidse 410 220 



Paludinidse 217 HO 



Calyptraeidae 160 101 



Turbinidse 855 906 



Haliotidge 104 136 



FissureUidas 201 76 



Recent. 



Neritidee 428 



Patellidse 368 



Dentaliadse 50 



Chitonidse 250 



8,465 

 Pulmonifera. 



Helicidse 4750 



Limacidse 93 



Limneeidse 332 



(Mirine) 193 



(Ditto, shell-less) 36 



6^404 

 Operciilated Pulmonifera. 



Cyclostomidae 903 



Aciculidse 28 



931 

 Tecti-branchiata. 



Tornatellidae 62 



Bullidee 168 



Aplysiadee 84 



Pleurobrancliidge ...... 28 



Phyllidiadce ..••• 14: 



356 

 Nudibranchiata. 



Doridee 160 



Tritoniadae 38 



.SEolidae 101 



Phyllirhoidse 6 



Elysiadse 13 



318 



Fossil 



103 



104 



125 



37 



5,819 



316 



4 



185 



37 



642 



45 

 1 



46 



166 



88 



4 



5 



263 



*Burchell, in Darwin's Journal, p. 87. 



\ Including Aporrhats 



X With Scalaria, 



