54 NEW YORK STATE MUSEUM 



b e c k i , were it not for the great variability of the exterior, and 

 perhaps in all respects its structural affiliations are closest with that 

 shell ; but it never attains the size of that species, while its persistent 

 small size, forms one of its most stable characters. 



Shell of medium size, very gently concavo-convex. Hinge-line 

 straight, attaining the greatest width of the shell ; at full growth very 

 slightly produced at the cardinal extremities, but in young stages with 

 quite decided extensions. Cardinal area narrow and mainly confined 

 to the pedicle- val ve ; striated horizontally and crossed vertically by 

 ridges which make themselves apparent on the hinge-line as denticula- 

 tions. The area is crossed by a narrow delthyrium which is generally 

 covered. Surface of the pedicle-valve covered with fine, rounded striae 

 which increase rapidly by intercalation. At intervals on the surface, 

 usually distant, are sharply defined concentric varices or growth- 

 lines. Accompanying these are concentric corrugations, sometimes 

 so pronounced and regular that the surface suggests that of L e p - 

 taena rhomboidalis,' but generally finer and irregular. These 

 may extend over the entire surface or be best developed in the um- 

 bonal and median parts of the shell; or they may be very obscure 

 and often, when the concentric varices are frequent and strong, may 

 be altogether indistinct. Along the hinge-line the wrinkles are ob- 

 lique, being parallel to the extended extremities of early growth stages. 



On the brachial valve the ornament is of quite the same charac- 

 ter. The apophyses and scars of the interior are those prevailing in 

 this genus. The cardinal process is bifurcated, and the posterior face 

 of each division is concave. From the base of this process diverge 

 short, lateral ridges which are highly pustolose, between them lying a 

 shorter median ridge. In both valves the muscular scars are indistinct. 



Dimensions. Full grown shells seldom exceed a length of 20 mm 



and a width of 25 mm. 



Brachyprion majus, sp. nov. 



Plate 8, flg. 8-13 



1892. Stropheodonta, sp. n. A. Clarke, op. cit. p. 413 



There are two species of this genus, Brachyprion, in this 

 fauna, and both present some similarities to the species of the Helder- 



