I/O 



surface by the co-operation of the median convexity of the valve and the sinus- 

 of the niarg-in. 



The surface of both valves is ornamented with a great number of radial' 

 striae in alternation with as many interspaces. The striae gradually widen, and 

 increase their number by the interpolation of new ones at various distances 

 from the beak on the one hand, and on the other by dichotomy, the latter 

 being confined to, or at least predomonating in, the anterior part of the shell.. 

 The width of the striae is in no way constant even in specimens of nearly the 

 same magnitude : in some specimens they are prominent, while in others they 

 are rather low. The interspaces are somewhat narrower than the striae them- 

 selves. The width of the latter varies from about 1/3? mm. to 1/2 mm. 



Beside these radial striae there is a smaller number of concentric undula- 

 tions on tlie shell surface which look like so many concentric stria- 

 tions. They are especially abundantly developed in the marginal portion of 

 the sliell. Inside each of these concentric lines the radial striae widen, though- 

 very slowly, and on passing over it become somewhat narrower again : the 

 concentric hnes are in this way far more conspicuously exhibited than they 

 otherwise would have been. 



In a specimen in which the shell is almost entirely torn away something 

 of the interior structure is accessible (PI. XII., fig. 14). The adductors are 

 not very clearly impressed but the impressions of diductors are quite clear on 

 the cast of the interior of the ventral valve. Vascular impressions is partially 

 exhibited. Two vascular trunks start from beneath the impression of the 

 diductors on both sides and are divided into smaller vessels anteriorly 

 and laterally. The: ovaries are not visible in the present specimen, 

 probably because they are hidden underneath the divaricators. There seem 

 to be several remarkable differences with respect to the internal structures of 

 the Chinese fossil and those described as A. reticularis, for instance, by Hall 

 and Eastman. But these apparent differences are probably due to the fact 

 that while the impression of the Chinese fossil exhibits the aspect of the sur- 

 face of the mantle, those mentioned by Hall and Eastman show the impres- 

 sion of the structures insi<le the mantle. ' 



