139 



Sir R. MurcWson adduces as a proof of Lis theory of the non-existence 

 of fishes in the earlier seas, the large number of Cephalopoda in the SUurian 

 as compared with succeeding Palseozoic formations, one set of tyrants at a time 

 being considered by him amply sufficient to prey upon the smaller tenants of the 

 deep. It may be replied, however, that we have abundant evidence in the 

 Carboniferous and Liassic rooks of the coexistence of carnivorous Cephalopoda 

 and fishes. It is difficult to say which biological theories are weaker, those 

 founded on a priori considerations or on negative evidence. 



In the new edition of "Siluria" there are given 79 British species of 

 Silurian Cephalopoda, of which 8 only have been found certainly in both 

 divisions, 30 appear to be exclusively Lower Silurian, being most abundant in 

 the Caradoc beds, a few are confined to the intermediate Llandovery zone, while 

 the rest have been hitherto discovered only in Upper Silurian rocks. 



CLASS GASTEROPODA. 

 Order i.— Peosobraxchiata. 



(Siphonostomata) (Holostomata) 



Strombidse *Naticid8e (?) 



Muricidae CanceUariadae 



Buccinidae *P3rramidellidae 



Cassididos Solariadse 



Conidae Scalariadse 



Volutidae Cerithiadse 



Gypraeidaa *Turritellidae 



Melaniadse 

 Paludinidae 

 Litorinidse 

 *Calyptrseid86 

 *TurbLnid8e 

 *Haliotid» 

 *Ianthuiidae 

 FissureUidse 

 Neritidoe 

 •PateUidae (?) 

 Dentaliadae 

 *CMtonid» (?) 



Order ii. — Pdlmonifera. 



(Inopercalata) (Operculata) 



Helicidae Cyclostomidae 



Limacidae Eelicinidae 



Limnaeidae Aciculidae 



AuricuUdae 



b2 



