MOLLUSCA—CONULARIDA—HYOLITHID^. 



bending of the engirdling growth Hnes. The opening closed by a 

 concentrically striated operculum. Cambric-Permic. 



1. H. americanus BilHngs. (Fig. i2ii,a-J.) Cambric. 

 Growth lines curve forward on the dorsal side, pass downwards 



over the sides at nearly a right angle and curve slightly backward 

 over the ventral side. Shell wall thin. 



Lower Cambric of Newfoundland, Quebec, Massachusetts and 

 New York. 



2. H. billingsi Walcott. (Fig. 1211,^-^.) Cambric. 

 Transverse section subtriangular. Shell thick, composed of suc- 

 cessive layers. 



Lower Cambric of Labrador, Massachusetts (?), Utah and 

 Nevada. 



3. H. impar Ford. (Fig. 1211, h-k.) Cambric. 



Hyolithes princeps; d, operculum; e, section. (After Walcott.) 



Usual length, ij inches; one or more transverse imperforate 

 septa present near apex. Transverse section generally regularly 

 oval. Apical angle about 10°. Shell wall thick, consisting of an 

 inner layer irregularly separated from an outer one. 



Lower Cambric of Massachusetts and New York. 



