PAUEONTOLOGY OF NEW YORK. 



beaks of the two 



Diagnosis. Shells sub-equivalve, equilateral; elongate-ovate, sub-quadrate 

 or 8ub-triangular in outline ; broad over the pallial region, cardinal slopes more 

 or less conspicuous ; slightly gaping at both extremities. Brachial or dorsal 

 valve someu-hat the shorter, and with a slightly thickened hinge-line. Surface 

 of the shell smooth, or concentrically and radiately striated. Animal attached 

 by a long, muscular pedicle protruding from between the 

 valves. 



Muscular impressions numerous, but usually in- 

 distinct. In the recent species they are twelve 

 in number upon each valve, and are somewhat 

 unsymmetrical in their arrangement. Tliey may 

 be designated as follows : The umbonal impressions 

 (g), produced by a single muscular band passing 

 directly across the cavity of the shell near the 

 beaks, (ind by their contraction opening the valves; 

 the lateral impressions, which are produced by 

 three pairs of muscles, the anteriors {j) passing 

 from near the lateral boundaries of the visceral 

 area on the pedicle-valve, forward to the anterior 

 extremity of this tract on the brachial valve ; the 

 middles (k) passing in just the opposite direction, 

 from the anterior region of the pedicle-valve to 

 the lateral region of the brachial ; the externals 

 (l) passing from the ante-lateral region of the 

 pedicle-valve to the post-lateral regiou of tlie 

 brachial valve, these muscles serving to move the 

 valves forward and backward. Tlie central impres- 

 sions (A) are produced by a single pair of muscles extending across the ante- 

 lateral region of the visceral area, and b}^ the contraction of these, the valves are 

 closed. The transmedian impressions (z), are made by a triple muscle, one band 

 of which i.s on one side the visceral area, the other two on the other side, the 

 two lateral components crossing each other in passing from the posterior region 



KiG. ]. T.ini/ula anatina, after HANCOCK. 

 ;*, pedicle ; g, uinboiiMl muscle ; h, cen- 

 trals ; /, transniedians;,/, A:, Z, laterals (.7, 

 anleriors, *, middles, 2, externals); e, heart; 

 o, moulh; a, al'iuGntary canal ; «, anus. 



