OF CENTRAL CANADA PART IV. 279 



Two essentially palaeozoic forms, Tentaculites (Silurian, Devonian), 

 and Conularia (Silurian to Lower Jurassic), are commonly regarded 

 as thecosomatous pteropods, but their true position is still uncertain. 



FIG. 217. FIG. 218. 



Tentaculites Ornatua Conularia Trentonensis : 



Upper Silurian. Lower Silurian. 



The shell, in Tentaculites, is tubular, of narrow diameter, tapering 

 to a rounded point, and transversely ringed. In Conularia, it is 

 more or less conical and four-angled, but usually flattened by com- 

 pression. Its surface is marked by a few longitudinal furrows, and 

 by numerous fine lines, resembling rows of punctures, arrange 

 transversely in zigzag form as shewn in figure 216. Some examples 

 are several inches in length. 



CLASS IV. GASTEROPODA. 



In the mollusks of this class there is a more or less distinct head 

 and all are furnished with a radula or lingual ribbon, thickly set 

 with minute teeth. The number, form and arrangement of the latter 

 constitute valuable classification-characters as regards living species, 

 but are without, or of only indirect, palseontological application, 

 Typical gasteropods possess also a fleshy expansion or so-called foot 

 on the under side of the body, by which locomotion is effected : hence 

 the name of the class. The greater number secrete an external uni- 

 valve shell, mostly spiral in form, but some few, as the slugs, are 

 naked, or possess merely a rudimentary shell ; and in the chitons, an 

 exceptional group, the shell is composed of several pieces. In many 

 of the spiral forms, the mouth or aperture of the shell can be closed 

 when the animal has retired within it, by a shelly or horny plate, 

 known as an " operculum." Some gasteropods, as the common 



