NATURAL SCIENCES OF PHILADELPHIA. 'J 



rather large, very oblique, becoming distinctly arcuate upwards. Left valve 

 very ventricose, with a tapering, incurved beak, closely approximated to its 

 fellow ; body of valve regularly arched along the umbonal slope, from which 

 line it describes a rapid convexity to the anterior margin, sloping more gradu- 

 ally to the ventral margin and becoming gradually flattened toward the 

 posterior ventral angle. The upper boundary of the body is an abrupt 

 descent to the plane of the posterior wing, and sharply divides the two ; 

 posterior wing sloping to the dorsal and posterior borders of the valve, 

 produced above into a slender spine, nearly as long as the posterior end 

 of the shell, with a deep sinuation below. Anterior ear short, saccate, less 

 distinctly divided from the body of the valve. Hinge-line straight, with a long, 

 posterior cartilage facet. Surface marked by irregular wrinkles of growth 

 which become fine striae on the posterior wing, and sharp plications on the an- 

 terior slope and auriculation. Right valve smoother and considerably less 

 ventricose, with the posterior wing-surface divided from the body of the valve 

 only by a slight groove. 



Length from beak to extremity of posterior wing -81 ; from beak to extremity 

 of anterior wing -21 ; from middle of dorsal side to ventral side -70; greatest 

 width of body of shell -48 ; same width in a larger specimen -93 ; depth of right 

 valve of same specimen -30. 



An occasional specimen of this species, differing from the types of the above 

 description only in the absence of arcuation of the body, presents a good agree- 

 ment with Prof. Hall's figure and brief diagnosis, diverging only in the less 

 forward direction of the beak, in the much larger anterior ear, and deeper 

 byssal sinus beneath it. The prevailing forms greatly resemble Avicula lunu- 

 lata, Phil. sp. (Geol. Yorks, ii. pi. vi. fig. 12). It is, however, less oblique, es- 

 pecially in the earlier stages of its growth, and the beak is narrower and more 

 depressed. It is also considerably broader on the aatero-ventral side, and has 

 a larger posterior wing. 



AVICULOPECTEN, McCoy. 



Aviculopecten Caroli, n. sp. Shell of medium size, subcircular, ventri- 

 cose. Hinge-line shorter than the shell; anterior ear of right valve shorter 

 than anterior end of shell, rounded, slightly inflated, with a deep, sharply- 

 rounded notch below; posterior ear acute, slightly longer than the anterior, 

 with a broad, shallow notch below ; shell otherwise nearly equilateral. Beak 

 central, inconspicuous ; greatest convexity of valve a little above the middle. 

 External surface of the body of the valve marked by about 25 nearly equidis- 

 tant, narrow, sharply-raised, radiating ribs, with two or three fine, raised striae 

 in each of the interspaces ; a set of very fine, sharp, close, concentric raised 

 lines cross the smaller ribs, but are intercepted by the primary ones. The 

 latter, however, show a tendency, toward the pallia! margin of the valve, to de- 

 velope nodes, which, on the anterior and posterior slopes, become distinct spines. 

 The spines sometimes oceur in the spaces between the primary ribs. The 

 wings are also marked by two sets of raised lines, but on the posterior wing 

 the radiating set is most prominent, while on the anterior wing the concentric 

 set is strongest. The left valve is exactly like the right, except that the notch 

 below the anterior ear is shallower. 



Length from beak to ventral margin -66 (100) ; length of hinge-line -55 (83) ; 

 convexity of right valve -20 (30) ; antero-posterior dimension -6$ (100). Num- 

 ber of concentric lines in one-tenth of an inch, 16. The adult size of the 

 species seems to be about one inch in length and breadth. 



Ranges from the base of the yellow sandstones into the base of the Barling- 

 ton limestone. 



Aviculopecten occidektalis, n. sp. Shell small, appressed ; hinge-iine 

 equal to greatest width ; anterior and posterior umbonal ridges at right angles, 

 and straight to the middle of the shell extremities, between which the pallial 



1863.] 



