OF CENTRAL CANADA PART IV. 



moreover, are founded on very slight differences, and should rank as 

 varieties only. 



Si2)honostomata : The representatives of this section are essen- 

 tially carnivorous types. They are distinguished by having the 

 aperture of the shell knotched at both of its extremities, or 

 otherwise by the aperture being extended into a slit tube or so-called 

 canal. Their remains are apparently unknown in Paleozoic strata^ 

 but become abundant in Mesozoic and Cainozoic beds, and their 

 representatives are still more abundant in the 

 seas of the existing period. The more typi- 

 cal families comprise : the Cerithiadce, Pur- 

 puridce, and Fusidce, dating from the Triassic 

 period ; the Srombidce, uccinidce and Neri~ 

 neidce, dating from the Jurassic period ; the 

 Muricidce, Volutidce, Cancellaridce, Conidw, and 

 Cypraeidce,'d&ting from Cretaceous times. The 

 annexed figure represents a species of Buccinum 

 from the Post-Glacial, Saxicava-sand formation FIG. 226. 



nf Rpaiinnrt npqv Onhpp Buccinum undatum 



beauport, n L VJUI Post Cainozoic and living. 



CLASS V. HETEROPODA. 



The mollusks of this class are regarded by many zoologists as 

 simply an Order ( = Nucleobranchiata ) of the Gasteropods. They 

 are free-swimming, pelagic types, and thus resemble in habit the 

 Pteropods, but are of much higher organization. They possess a 

 well-developed head, with perfectly formed eyes, &c., and the sexes 

 are distinct. Some are without a shell ; others have a very small, 

 thin shell, protecting only a portion of the body ; and in those of the 

 Family Atlantidce, there is a light shell of sufficient size to contain 

 the entire animal, and this in some cases is furnished with an 

 operculum. 



Two families are commonly recognized : 1 , Firolidce (or Ptero- 

 trachceidce), in which natation is partly performed by a so-called "tail 

 fin," including the naked genus Firola, and also the Carinaria with 

 small, delicate, patelliform shell ; and 2, Atlantidce, in which there is a 

 light, hyaline shell into which the animal can withdraw its body. The 

 only known fossils are some small shells of two species of Carinaria, 

 from European strata of Miocene age. The extinct genera, Bellerophon, 



