98 TRANSACTIONS OF THE WAGNER FREE 



Length, 1.25 inches; height, from base to hinge-line, .5 inch. 



This winged ark is at once distinguished from A. aviculceformis ( 

 by the absence of the anterior rostrum and its rectangular form. The last 

 character, in addition to differences in the ornamentation, also serves to 

 distinguish it from the Miocene Area incilc, which resembles it somewhat 

 in the pterination of the posterior slope. 



ARCA. 

 Subgenus Arcoptera, Heilprin. 



Shell elongated, aviculaeform, rostrated anteriorly, winged posteriorly ; 

 hinge-line practically the entire length of the shell, exceedingly narrow, 

 and pectinated with a crowded series of transversely directed or partially 

 v-shaped teeth ; umbones widely separated ; ligamental area very broad, 

 open ; base sinuous, with a median opening. 



I propose this subgenus for a series of very remarkable arks, which 

 can be readily distinguished from all other members of the genus by their 

 peculiar pterination and rostration, giving an external outline precisely 

 that of Avicula. This character is accompanied by a most extraordinary 

 attenuation anteriorly of the chamber of the shell. 



Area (Arcoptera) avioulaeformis, nov. sp. Fig. 32. 



Shell elongated, aviculaeform, rostrated anteriorly, winged posteriorly, 

 with a prominent obtuse carination on the umbonal slope bounding the 

 wing; rostrum declivous, marked off from the body of the shell by a 

 broad hollow ; basal margin of shell sinuous, showing a median opening, 

 and rapidly sloping upward in the direction of the rostrum; posterior 

 border deeply emarginate. 



Umbones acute, very excentric, moderately elevated, and but slightly 

 incurved, with a gradual continuous slope to either extremity of shell ; 

 hinge-line nearly the whole length of shell, very narrow, pectinated with 

 a crowded series of lamellar, transversely directed, teeth, which exhibit a 

 tendency to become oblique and v-shaped on the posterior half of the 

 line ; ligamental area broad, open, arching upward in a gentle curve, 

 longitudinally lined, and irregularly grooved by numerous diagonal or 

 v-shaped furrows resembling insect borings. 



Surface of shell ornamented with numerous radiating wavy lines, alter- 

 nating in coarseness, which become more or less obsolete on the umbonal 

 slope, and are wholly wanting on both the beak and wing, which only 

 show concentric lines of growth ; of the radiating lines on the anterior 

 part of the shell the series runs about as follows : coarse line, followed 

 by two finer lines, then a slightly more prominent single line, again 

 two finer lines, and then a coarse line, same as first, marking the coarse 



