OF CENTRAL CANADA PART IV. 



287 



The genus Orthoceras is our most commonly occurring type of this 

 division. It presents a long, more or less conical shell, with simple 

 concave septa. The exterior surface of the shell is smooth or 

 delicately striated in most species ; but in some it is transversely 

 ringed, and in others marked with longitudinal ridges and furrows. 

 The Siphuncle is either central or excentric in position, and either of 

 small or large diameter. In the latter case it sometimes contains a 

 solid cone of calcareous matter, as shown in figure 232. Very 

 frequently it presents a so-called beaded form, expanded and con- 

 tracted at regular intervals (fig. 233), and this becomes especially 

 marked in certain forms from Lake Huron and elsewhere, regarded 

 by some geologists as forming distinct sub-genera under the names of 

 Actinoceras and Huronia. Figures of several of our characteristic 

 species, with examples also of other genera, are shown in the 

 annexed engravings. 



FIG. 229. 



FIG. 229. a. 



FIG. 230. FIG. 235. 



Explanation of figures : 



Fig. 22.'. Phragmoceras Hector. (Billings.) Guelph Formation. 

 229 a. Aperture of P. Hector. 



Fig. 230. Lituites undatus (Hall). Lower Silurian. 

 Fig. 235. Ascoceras Townsendi (Whiteaves). Guelph Formation, 

 Ontario. 



