Compared 

 with other 

 species. 



188 



front of which two narrow diverging ridges run forward to the front of 

 the valve, where they are about as far apart as one-quarter of the length of 

 the valve. There is considerable variation in the size and position of the 

 foramen in the example collected. Interior. The foraminal passage is 

 smaller within than at the outer surface of the shell, and is surrounded 

 by a "raised rim ; from it two thread-like grooves run forward into the 

 front of the umbonal cavity. (The ridges that run forward from the 

 foramen on the outer surface of the shell are preserved as grooves on its 

 inner surface.) The visceral callus, of a lenticular outline, extends about 

 half as far in front of the foramen as that is from the posterior margin ; 

 it is crossed by two faint diverging ridges on each side, and is bordered 

 by two stronger lateral ridges, widely diverging, that mark the position 

 of the lateral muscles ; the length of these ridges is about one-third of 

 that of the shell. Faint traces of vascular trunks are found in the lateral 

 and posterior part of the shell and make a regular arch about one-sixth 

 of the length of the shell, from its margin. The margin is flattened, 

 especially in the posterior half, toward the umbo. 



The dorsal valve is flatter than the ventral, and its umbo somewhat 

 removed from the posterior margin. The central part of the valve has 

 a flattened triangular space extending back towards the umbo ; as the 

 lateral margins are flattened in the posterior half, a low flattened ridge 

 extends out on each side from the umbo to the mid-length of the valve. 

 Interior. This shows a broad flattened mesian ridge extending half way 

 across the valve from the posterior margin, on each side of this, about 

 one-third from the back of the valve, and nearly as far apart, are 

 obscure oval marks, probably indicating the position of the anterior 

 adductor muscles. The margins of the ralve are flattened behind. 

 Sculpture. This consists of sharply defined but very minute concentric 

 and radiating ridges that form a delicate cancellated pattern ; on the 

 highest part of the shell the concentric ridges are most distinct, on the 

 front part, the radiating ridges. No cicatrix marking the advance of the 

 foramen was observed, but a progressive change of this kind is pro- 

 bably indicated by the paired thread-like ridges behind the foramen on 

 the interior of the ventral valve. 



Size. Length and width each 4 mm. Depth about of a millimeter, 

 that of the dorsal valve less. 



Horizon and locality. Fine dark gray shales of the Dictyonema beds 

 (C. 3c) at McLeod brook, Cape Breton. 



This pretty little species is the smallest and oldest known of its genus. 

 Mr. Walcott indicates for S. typicalis a calcareo-corneous shell,* but 



*U. S. Geol. Surv. Monogr. viii, p. 70, pi. 1, figs. 3, 3a to c. 



