MINERALS AND GEOLOGY OF CANADA. 



119 



the class G-asteeopoda. They constitute however a truly aberrant 

 group, having affinities with the Pteropods on the one hand, and 

 with both Q-asteropods and Cephalopods on the other. Existing 

 forms, like the pteropods, are of pelagic habit, swimming, by means 

 of a fin-like appendage, in the open seas. The swimming organ is a 

 modification of the gasteropod foot : see below. Some are without 

 a shell, whilst others secrete one of a fragile and delicate texture. 



Fig. 118. 



Fig. 117. 



sometimes provided, as in many gastero- 

 pods, with a lid or " operculum," by 

 which the opening of the shell is closed 

 when the animal withdraws itself with- 

 in it. The fossil genera Maclurea, 

 Bellerophon, and Cyrtolites, from certain 

 characters which their shells appear to '^* 

 possess in common with those of the modern genus Atalanta, are 

 usually referred to this class ; but much uncertainty still prevails 

 with regard to the true affinities of these fossil types. The compara- 

 tive solidity of the shell is opposed to their alliance with the Atalan- 

 tidcB. Mr. Salter of the English G-eological Survey, suggests, how- 

 ever, that Maclurea may have been a Heteropod with heavy shell, 

 inhabiting the sea-bottom. Fig. 117, represents Maclurea Logani 

 of the lower part of the Trenton Group ; o is an inside view of the 

 curious operculum often found detached. Fig. 118 is an example of 

 JBellerophon expansus, and fig. 119 of Cyrtolites ornatiis, of the 

 Trenton and Hudson River Groups (Lower Silurian series.) By 

 some palaeontologists, the genera Bellerophon and Oyrfolites are 

 considered identical. 



Q-ASTEEOPODA. — The gasteropods have a distinct head ; and all 

 the typical species possess a fleshy expansion or foot on which they 

 creep, and from which the class derives its name. The greater 



