Lamellibranchiata. 28? 



and merged with the finer striae. The pallial line originates in a 

 small, deep, anterior muscular impression just beneath the anterior 

 tooth, and extends nearly parallel with the margin of the valve, 

 almost two-thirds the distance from the beak to the base of the 

 shell, where it is abruptly recurved, terminating in a large, ovate, 

 posterior muscular impression directly below the posterior extremi- 

 ties of the lateral teeth. Ligamental area marked by a variable 

 number of sharp, narrow grooves, which extend to the extremities 

 of the hinge-line, and probably corresponding to the successive 

 laminae of growth. In the left valve, there are four or five linear, 

 diverging cardinal teeth beneath and anterior to the beak ; in the 

 right valve only two or three such teeth are seen. Two or three 

 linear, slightly diverging, lateral teeth, are situated below the liga- 

 mental area, and posterior to the umbo. 



A young symmetrical specimen has a length and height (without 

 the ear) of 15 mm.; and the greatest extent from the beak to the 

 post-basal margin is 21 mm. A medium sized specimen is 36 mm. 

 in vertical height ; extreme distance from beak to base 47 mm. ; 

 length parallel to hinge-line 35 mm., and hinge-line from beak to 

 extremity of wing 38 mm. Another is 46 mm. in vertical height; 

 extreme distance from beak to base 52 mm. ; length parallel to 

 hinge-line 46 mm., and hinge-line from beak to extremity of poste- 

 rior wing 56 mm. A specimen of different proportions is 52 mm. 

 in vertical height, extreme distance from beak to post-basal margin 

 63 mm., length parallel to hinge-line 40 mm., hinge-line from 

 beak to extremity of posterior wing 40 mm. 



This species in surface ornamentation differs greatly from every 

 other form below the carboniferous period, and possesses true 

 pterinoid characters in a stronger degree than any other species 

 from the New York rocks. The right valve when detached is not 

 so easily distinguished from some other forms, as Pterinea Chemung- 

 ensis and Actinoptera Boydi. 



The species is co-extensive with the Hamilton group throughout 

 the State of New York. An apparently abnormal form, in its un- 

 usually large wing, found in the Chemung group, has been referred to 

 this species ; and specimens of similar character are common in certain 

 localities. 



The specimens occurring in the Chemung group are much larger, 

 and of more extravagant growth, than any yet observed in the 

 Hamilton group. A large specimen has a height of 100 mm. ; another 

 measures 90 mm. in length, and is of equal extent along the hinge-line. 

 The costae on these large forms are often only four in number and 

 very strong and broad. 



