palj:ontology of new york. 



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

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

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

 valve somewhat 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 beaks of the two 

 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. They may 

 be designated as follows : The umbonal impressions 

 (g), produced hy 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 J, 

 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 

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

 pedicle-valve to the post-lateral region of the 

 brachial valve, these muscles serving to move the 

 valves forward and backward. The central impres- 

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

 lateral region of the visceral area, and by the contraction of these, the valves are 

 closed. The transmedian impressions {i), are made by a triple muscle, one band 

 of which is 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 



Fig. 1. Lmgnla anatinn, after Hancock. 

 />, pedicle ; g, umbonal muscle ; ft, cen- 

 trals ; I, traiisnie(lians;,y, A-, /, laterals (.?', 

 anteriors. t, middles, I, externals); c, heart; 

 o.moulh; a, alimentary canal ; z, anus. 



