156 PALAEONTOLOGY OF NEW YORK. 



appear to be sometimes complicated by association with ill-defined scars of the 

 anterior muscles. The posterior adductors or divaricators are situated at the 

 basal angles of the muscular triangle, and are distant from the posterior mar- 

 gin. The linear parietal scars are very strong, the posterior being more or less 

 distinctly lobate, the anterior generally straight or rounding about the central 

 adductors. In the opposite or dorsal (?) valve the scars have essentially the same 

 arrangement; the anterior adductors, however, are separated by elongate median 

 scars (anteriors) which traverse the elevated callosity surrounding the anterior 

 margin of the area. The posterior scars are often more widely divergent than 

 in the other valve. Shell-substance calcareous and impunctate (?). 

 Type, Pholidops squamiformis, Hall. 



Observations. This peculiar group of shells presents an interesting associa- 

 tion of features which, so far as known, is susceptible to slight variation. The 

 character of their muscular anatomy is distinctly cranioid, as seen in the 

 development of the two strong pairs' of adductor scars, but the concentration 

 of the muscular impressions and the resulting removal of the posterior scars 

 forward from the margin is a feature not seen in any Crania. ; added to this 

 is the usually sharp impression of the parietal wall which is rarely observable 

 in any of the inarticulates except the Trimerelloids. The posterior lobate 

 limb of this impression in Pholidops is suggestive of the scalloped or sinused 

 arch seen in Lingulops,* but we should hesitate to suggest an analogy between 

 the two. 



The depth of the muscular impressions in Pholidops, evidently an index of 

 the strength of the muscular bands, is a natural result of their concentration 

 within the confined visceral area, for by such a displacement a great advantage 

 in the closing of the valves is sacrificed. 



In external features, outline and contour, there occur some noticeable varia- 

 tions. The typical species, P. squamiformis, with oval outline and subcentral 

 beak, represents the character of exterior prevailing among the species 



* In discussing the genus Lingdlops, we have indicated that the scar of the parietal wall constitutes 

 the crown of the crescent and that thei-e is no satisfactory i-eason for ascribing to the crescent in Lingulops 

 one function, and to that in Tkimbrella another, as was done by Davidson and King. 



