54 



PALEONTOLOGY OF NEW YORK. 



depression of the surrounding surface of the shell is never so marked ; the 

 platform itself is of less size throughout, and we have noticed that in the course 

 of its development it does not make its appearance so early as in the brachial 

 valve. The lead held by the brachial valve in this respect, as shown in Lingu- 

 LOPS and Elkania, is maintained throughout the entire history of the organ. 

 The cause of this is not far to seek. In Lingula, the anterior portion of the 

 visceral region lying directly in front of, and 

 behind the insertion of the anterior muscular 

 bands, is occupied by the liver. In L. anatina, 

 rather the greater portion of this body lies 

 behind these muscular scars, but in Lingulops 

 there is a general posterior concentration of all 

 these muscular bands. In such a case the retro- 

 gression of the muscles could not force the liver 

 into a more contracted space or otherwise 

 displace it than to change its relative position 

 and leave the large muscular bands behind it. 

 This organ (the liver) is large, and lies mostly 

 on the dorsal aspect of the animal, the surface 

 there covered by it, in L. anatina, being con- 



. . Fig. 17. Lingula anatina. 



siderably more extensive than on the opposite Ab oiai aspect, showing the. anangemeuiufihe 



.. _. , T- iiMiTi 1 • viscera on the inner surface of the brachial 



Side, it is also distinctly bilobed by tiie longi- vaive (.after king). 



j.j-1 J. ii,T_i-i 1 mu ft, central scars. a, liTer. 



tudmal septum on the brachial valve, ihe j, .interior lateral 

 pressure of this organ against the anterior edges 

 of the area of muscular implantation must have been of significance in induc- 

 ing the formation of the anterior elevated margins of the muscular scars, and 

 in this pressure, continued as a constant force in both valves, though less in- 

 tensely in one than in the other, may, perhaps, be found an efficient cause for 

 the abrupt termination of the muscular area, its elevation, its concave anterior 

 walls, and, eventually, its complete excavation, as well as the depression of the 

 adjoining surfaces of the shell. 



In discussing the genus Dinobolus, attention has been directed to the tendency 



h, genitalia. 



e, aliraentarv canal. 



