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dorsal valve a slightly elevated median line occurs at the bottom of the 

 groove, between the central muscle scars, that extend forward to the 

 anterior margin of the visceral cavity, beyond the anterior lateral muscle 

 scars. The visceral cavity of the ventral valve extend forward to about 

 the centre ; in some shells it is back of the centre, and in others a little 

 in front. It varies in width and outline very much as the shells vary, 

 being wide in broad shells, and narrow in elongated forms. 



Description of Vascular system. The markings left on the shell by the vascular system 

 are v< ry strong, and beautifully preserved in some portions. In some 

 shells there is a double groove with a slight ridge between ; in others the 

 ridge is large, only a trace of an outer groove remaining ; in some young 

 shells, the groove is broad and shallow ; in all shells the large size of the 

 main vessels is shown by the broad, strong grooves or ridges left on the 

 shell. It frequently happens that the deeply indented lines of pits on the 

 lines of growth deeply indent the grooves and rounded ridges, left by the 

 main vessels, and mark them off into sections. The interior and lateral 

 vessels left narrow but strong grooves or ridges on the shell, which how- 

 ever are usually obscured by the strong pitting of the surfaces. 



The parietal scar surrounds the visceral cavity in each valve, crosses 

 the course of the main vascular vessels, and comes back around the spaces 

 occupied by the muscle scars, terminating at the edge of the area at the 

 flexures in the ventral valve ; termination unknown in the dorsal valve. 



Some of the muscle scars are finely shown in the dorsal valve and fairly 

 well in the ventral. The umbonal scar of the ventral valve is divided, 

 the pedicle scar being situated between the two parts. In the dorsal 

 valve the umbonal scar is close to the area, and extends nearly as far 

 each side of the median line as the length of the area. 



The scars of the central muscles in the ventral valve are crowded in 

 with the middle and outside laterals within the trapezoidal space. In 

 the dorsal valve, they are located on a low ridge each side of the central 

 longitudinal depression ; they are elongate oval in outline, their major 

 axis being sub-parellel to the median line of the shell ; fine longitudinal 

 lines cross the scars in the best preserved specimens ; the ridge on which 

 the central scars occur varies in strength, but it appears to be present in 

 all adult shells ; it narrows gradually posteriorly and rather rapidly to 

 the inner side of the anterior lateral muscles scars. The anterior laterals 

 of the ventral valves, are placed well back on the narrow space between 

 the edge of the area and the main vascular sinus ; they are elongate and 

 rather large ; in the dorsal valve they are elongate, with the major axis 

 inclined forward toward the median line. The middle and outer laterals 

 are situated in the trapezoidal area of the ventral valve, but neither is 





