264 



MOLLUSCA. 



of the Lower Silurian series both in North America and in 

 Scotland, occurring in some localities in the greatest profusion. 



Fig. 232. Mad-urea crenulata. a Spire ; b front , c base. Lower Silurian. 



The genus Ophileta (fig. 233) of the Silurian Rocks may be 

 mentioned here, though its true affinities are extremely doubt- 





Fig. 233. Ophileta bella (Billings). Different views of a nearly perfect specimen. 

 Quebec Group (Upper Cambrian ?) 



ful. The shell in this genus is discoidal, and very closely 

 resembles that of Eiwmphalus. The aperture, however, is 

 stated by Mr Billings to have a sinus in the 

 lower lip and a notch in the upper lip- 

 characters which are not present in Mac- 

 hirea. It is a matter of question whether 

 Ophileta should be regarded as comprising 

 species of Machtrea with slender whorls, 

 or whether it should be placed in the Tur- 

 binidce, in or near Euomphalus, or whether 

 it should not be placed in the Haliotidcz and 

 be regarded as a discoidal Pleurotomaria. 



In the genus Cyrtolites (fig. 234) the shell 

 is thin, symmetrical, discoidal, or coiled into 

 the shape of a horn, the whorls more or less disconnected, 

 furnished with a keel, and sculptured. The species of this 



Fig. 234. Cyrtolites 

 ornatus. Lower Silu- 

 rian. 



