286 DAVEXPORT ACADEMY OF NATURAL SCIEXCES. 



Whitfield. From the latter it differs in size, being much smaller, having 

 a less ventricose aspect, a finer character of plications, a fewer number, 

 both on the sides and on the mesial sinus, while in B. Emmonsi the 

 breadth of the sinus is to the breadth of the shell as two to three, in our 

 species the breadth of shell is nearly twice the breadth of sinus. It dif- 

 fers from It. venustula. Hall, in some of the same particulars. It is 

 smaller, less ventricose, and more finely plicated. The youngest speci- 

 mens yet found maintain the same general form as the older, the plica- 

 tions not being confined to the middle portion, but extending along the 

 side. In a smaller specimen than any figured by Prof. Hall, there are 

 eight plications on the mesial fold (while this is the full number ascribed 

 to an adult of R. venustula), forty may be counted on the sides. In ordi- 

 nary sized shells the number of plications is still greater,— on the sides 

 about fifty, on the mesial fold from ten to twelve. The fold is more 

 square, reaching a higher elevation in front, measuring nearly the height 

 of the shell. The plications are not flattened as in R. venustula, but 

 rounded. The groove longitudinally dividing each plication is armed 

 with a pinnatified structure, which is exhibited on the sides as well as on 

 the front of the shell. In our specimen a still deeper groove separates 

 the plications from each other, and forms a conspicuous feature. This 

 shell is found in bed Xo. 2, a dove-colored compact limestone. It is not 

 abundant. The specimen figured is larger than the ordinary size. 



Fig. 5 represents a cardinal view of shell. 



Fig. 6. — Front view, with ten plications in the sinus, showing pinnati- 

 fied structure. 



Fig. 7.— Profile view. This shell is found only in bed No. 2. Specimen 

 figured with others presented to the Academy by the writer. 



LAMELLIBRAXCHIATA. 



Avicula (Plerinea) cancellata (n. sp.). 



Plate xi, fig. 9. 



Shell obliquely subovate, length twice the breadth, gradually expanding 

 from the beak. The right valve convex, marked by concentric striae 

 and apparent lines of growth. The left valve is far more convex than 

 the right, with concentric strise of very unequal strength, which are 

 crossed by radiating strise, the concentric predominating over the 

 radiating in numbers, strength and persistency. • The hinge line is 

 straight. The beaks are oblique, that of the left valve very convex, that 

 of the right projecting a little above the hinge margin. The posterior 

 wing, so far as shown— nearly the length of the shell — marked with sim- 

 ilar unequal stvite as on the body ; the anterior wing not shown on any 

 specimens. 



This shell has been found in three conditions. First, as a cast from 

 which every trace of marking has disappeared, next as in the case of a 

 specimen in the cabinet of Griswold College, where both valves are pre- 

 served, and yet only showing concentric striae ; and as in the present 



