CLINTON, LOWER IIELDERBEIUJ. CIIEMUNO, AND MAVEKLY CUOUPS, 447 



LEPTODESMA LEIOPTEEOIDES Simpson, n. sp., Fig. 13. 



Shell above the medium size, subrhomboidal ; body elongate ovate, oblique at an 

 angle of about fifty degrees to the hinge line; height a little less than two-thirds the 



Vlllg.. 



\ 



\ 



'-> 



1 



^-- 



A'r.S. trl'iis. 



length ; antebyssal margin rounded, much constricted at the b3'ssal sinu.«, then broadly 

 rounding, somewhat abruptly recurving at the postbasal extremity. 



Left valve gibbous above the middle, becoming flattened on the jjostero- basal 

 portion. Kight valve unknown. 



Hinge line straight, length unknown, as a perfect specimen has not been observed. 



Beak subanterior, prominent, oblicjue, directed forward; uiiihonal region gib- 

 bous, descending more abruptly on tin; posterior than on the anterior side. 



Anterior end short, angular at the extremity, rounded below, separated from the 

 body by a distinct, nearly vertical byssal depression. Wing narrow-, triangular, 

 joining the body at a little more than one-third the length of the valve from the 

 postei-ior extremity ; margin oblique below, becoming deeply concave and abruptly 

 curving backward, and probably produced in a spiuiform extension. 



Siufaee marked by concentric stria' which are most distinct on the wing near the 

 body, being often fine, sharp, and lamellose in appearance; frequently on the body 

 becoming fasciculated and producing an undulated surface. The wing is sepa- 

 rated from the body by a well-delined, naiTow groove, which regularly curves inward 

 from the beak to the junction of the wing with the body. 



The specimens vary in pro|)ortional height and length ; two left valves having re- 

 spectively a length of GO and 50 mm. and a height of 40 and 45 mm. The width 

 of the body of the longer specimen at the junction of the wing with the body is 30 

 mm., of the shorter one 35 mm. 



The specimens usually have somewhat the appearance of the genus Leiopteria. 



Some of the specimens resemble Leptodesma billinf/si of this foiniation, l)Ut may 

 be distinguished from that sjjecies by the greater width of the lower ])ortion of the 

 body, the larger wing and the very distinct separation of the body and wing. 



