302 Thiety-eifth Report ok the State Museum. 



Pallial line extending parallel to the margin of the shell and termi- 

 nating in a muscular impression on the posterior slope. A small 

 muscular impression is also seen just in front of the beak, and ob- 

 scure indications of one or two lateral teeth. Ligamental area 

 narrow, striated, marked by two or three slender grooves, which are 

 slightly divergent from the hinge-line. 



One of the original specimens of A. quadrula (==A. Boydi) has 

 a length of 30 mm., height and hinge-line each 28 mm. A similar 

 specimen has a length of 25 mm., height and hinge-line each 23 

 mm. Two large specimens belonging to the group of A. Boydi 

 have the following respective dimensions : length 38 and 40 mm., 

 height 38 and 35 mm., hinge-line 34 and 30 mm. 



This species, in older specimens, resembles A. subdecussata, but 

 differs in the character of the rays, in the less obliquity, and the wider 

 body. The left valve somewhat resembles the right valve of Avicula 

 reticulata, Hisinger, but a comparison of the right valves of both 

 species shows them to be different. The forms here classed under one 

 specific designation were described by Mr. Conrad under different 

 species, viz. : Avicula Boydii, A. quadrula, A. pleuroptera, and prob- 

 ably A. perobliqua. 



The species in its various stages of growth and state of preserva- 

 tion presents a great variation in form and surface characters. The 

 usual condition is that of casts of the interior or specimens more or 

 less exfoliated, leaving the surface markings subdued or obsolescent, 

 and in a few specimens only, have the entire characters of the test in 

 either or both valves been observed. The older specimens show an 

 extension of the shell along the post-basal margin which is marked 

 by concentric strise and undulations, while the rays become interrupted 

 and obsolete. After a study and comparison of a very large number 

 of individuals, it has not been found possible to draw any satisfactory 

 line of specific distinction. 



Formation and localities. In the shales of the Hamilton group ; 

 abundant at numerous places in the eastern and central portions of 

 the State. 



Actinopteria perobliqua. 



Avicula per obliqua, Conrad. Jour. Acad. Nat. Sci., Phila., vol. viii, p. 235, pi. 



12, fig. 1. 1842. 

 Actinopteria perobliqua (Conrad), Hall. Pal. N. Y., vol. v, pt. 1. Plates and 



Explanations : PI. 19, fig. 31. Jan., 1883. 



Shell of medium size, sub-rhomboidal ; body very oblique, narrow- 

 ovate ; length one-third greater than the height ; anterior margin 

 truncate or slightly concave ; basal margin regularly arched and 



