84 



PAL^ONTOLOGT OF NEW-VORK. 



473. 7. AVICULA RIIOMBOIDEA (?.. v/).)- 



Pl. XXVII. Fig. 2o, b, c, d. 



Shell rhomboidal, liiohcr than long; hinge-line nearly straight, or slighll} ikllecUil ; base 

 rounded ; sides nearly parallel, and slightly sinuous ; beak prominent, and a little elevated 

 above the cardinal margin ; umbo prominent, becoming gradually depressed towards the mar- 

 gin ; anterior wing short, rounded, and separated from the central part of the shell by a sinus ; 

 anterior wing triangular, not exteiuling so far as the posterior margin of the shell ; surface 

 marked by concentric striaj, which become, at intervals, prominent folds indicating stages of 

 growth. 



Li young specimens the beak is usually more sharply defined, the umbo more prominent, 

 and sometimes the posterior wing is very acute. The specimens known occur mostly in sand- 

 stone, so that little more than a cast is preserved. In these casts there is no evidence of longi- 

 tudinal stria;, antl in a single specimen in shale there is likewise no evidence of such striae. 

 In sandstone the specimens are very convex, and even gibbous, while the one in shale is 

 extremely compressed, giving a slightly different aspect. 



Fig. 2 a. A large individual, in which the anterior wing is less prominent than usual. 

 Fig. 2 i. A smaller specimen. 



Fig. 2 c. A young specimen, having the length nearly equal to the height. 



Fig. 2 d. The posterior portion of another specimen, having the stria; more perfectly preserved 

 and the angle of the wing more extended. 



Position and localities. In the fine-grained sandstones near the base of the group at Black- 

 stone's quarries, New-Hartford ; and in the upper green shale of the group, associated with 

 CiioNKTES, at Sod us, Wayne county. 



474. 19. MODIOLOPSIS SUBALATUS («. i^;.). 



Pl. XXVII. Fig. 5, and 6 a, b. 



General form ovate or sub-rhomboidal ; posterior side expanded and sub-alate ; umbo pro- 

 minent, and the beak slightly elevated above the cardinal line ; anterior side narrow, rounded 

 or sub-acute ; surface marked by fine equal concentric stria;. 



This shell is usually of the size of those in fig. 5, of which considerable numbers occur at 

 Rochester in the upper green shale, and particularly in a thin band of purple shale included in 

 the former. It is associated with JJtrypa hemispherica ; but from its small size and compressed 

 condition it scarcely attracts attention. 



Fig 5. A fragment of slate, with figures of the right and left valve. 



Fig. 6 a, b. Figures of the right valve of two specimens of large size. The specimen fig. 6 a 



is somewhat crushed, and the figure gives an incorrect representation of its true 



character. 



