250 NEW YORK STATE MUSEUM 



From the above we see that it is only the specimens called by Hall 

 " large " that have a sufficient number of plications to strictly come 

 under the species according to his description. Large specimens 

 both from Becraft mountain and Trilobite mountain may be placed 

 in it but the great majority are comparatively small shells with an 

 average of nine plications on each side of the sinus. Taking the 

 large shell as the normal, the majority, both at Becraft and Trilobite 

 mountains, represent immature development, for the large shells 

 have passed through this stage as seen by taking younger stages on 

 them. 



In other respects the shells are very similar, they are quite strongly 

 incurved and gibbous at the umbo, the cardinal area is high, concave 

 and usually equals the greatest width of the shell. The sinus of the 

 smaller shells is not as angular as in the larger ones and in this 

 respect approaches S. cyclopterus. The similarity to this 

 latter species is more clearly shown in the young. The pedicle valve 

 of S. c o n c i n n u s , measuring 6mm by 8mm, has five plications 

 and a S. cyclopterus, 5mm by 9mm has four, but in the 

 latter species the plications are almost as pronounced as in the mature 

 shell, while on the former they are exceedingly faint. The con- 

 vexity of the two shells is very similar. Notwithstanding the close 

 resemblance of these two species, they can hardly be very closely 

 related since the characteristic plications of each persist from the 



youngest stages. 



S. murchisoni Castelnau 



HalP speaks of the great similarity between S. murchisoni 

 and S. cyclopterus and says that the former may be perhaps 

 only a variety of the latter '' which in the sandstone attains a larger 

 size than in the shaly limestone below." The young of S . 

 cyclopterus is very like that of S . murchisoni, many 

 of the former having angular cardinal extremities similar to the 

 latter. The hight and concavity of the cardinal area as well as the 

 number of plications and the surface markings are also alike. But 

 there seems to be a slight but constant, greater incurving of the 

 pedicle valve in the former. Of course with the mature shell there 



Tal. N. Y. 3:430. 



