318 SYSTEMATIC PALEONTOLOGY 



'-'. order OPISTHOBRANCHIA 



Suborder PTEROPODA 



Family HYOLITHIDAE 



Genus HYOLITHES Eichwald 



HYOLITHES COMMUNIS Billings 



Plate XXV, Figs. 5-8 



Hyolithes communis Billings, 1872, Canadian Nat., n. s., vol. vi, p. 214. 

 Hyolithes communis Walcott, 1886, Bull. U. S. Geol. Surv., No. 30, p. 136, 



pi. xiv, figs. 3, 3a-c. 

 Hyolithes communis Walcott, 1890, 10th Ann. Rep. U. S. Geol. Surv., p. 620, 



fig. 65, pi. Ixxvii, figs. 3, 3a-g. 



Description. " This species attains a length of about 18 lines, although 

 the majority of the specimens are from 10 to 15 lines in length. The 

 ventral [dorsal] side is flat (or only slightly convex) for about two-thirds 

 the width, and then rounded up to the sides. The latter are uniformly 

 convex. The dorsum [ventrum], although depressed convex, is never 

 distinctly flattened, as is the ventral [dorsal] side. The lower lip projects 

 forward for a distance equal to about one-fourth or one-third the depth 

 of the shell. In a specimen whose width is three lines, the depth is two 

 lines and a half. 



" The operculum is nearly circular, gently but irregularly convex 

 externally and concave within. The ventral [dorsal] limb is seen on the 

 outside as an obscurely triangular, slightly elevated space ; the apex of the 

 triangle being situated nearly in the center of the operculum. The base of 

 the triangle forms the ventral [dorsal] margin. This limb occupies about 

 one-third of the whole superficies of the external surface. The remainder, 

 constituting the dorsal [ventral] limb, is nearly flat, slightly elevated 

 from the margin towards the center. On each side of the apex of the 

 ventral [dorsal] limb there is a slight depression running from the nucleus 

 out to the edge. On the inside there is an obscure ridge corresponding to 

 each one of the external depressions. It is most prominent where it 

 reaches the edge. These two ridges meet at the center and divide the 

 whole of the inner surface of the operculum into two nearly equal 

 proportions. 



