LAMELLIBRANCHIATA. 95 



Ligamental area marked by a variable numlier of sharp narrow grooves, 

 which extend to the extremities of the hinge-line, and probably correspond 

 to the successive laminfe 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 ligamental 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 posterior 

 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 Chemungensis and AcUnoptera Boydi. 



The species is co-extensive with the Hamilton group throughout the State 

 of New York. An apparently abnormal form, in its unusually large wing, 

 (PI. XV., fig. 6,) 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. (See plate Ixxxiii, fig. 11.) 



