Report of the State Geologist, 579 



excenl ric or marginal, sometimes terminaJ and produced. Surface 

 marked by strong concent lie, often laniellosc lines of growth, 



which are crowded on the posterior, and distant on the anterior 



portions of the valves; these are sometimes crossed by faint 

 interrupted radiating lines. On the interior, the surfaces of con- 

 tact make a broad, smooth, flat or slightly convex border, some- 

 what broader in front than behind. The muscular and visceral 



area occupies a sharply defined and very limited space in the 

 apical portion of each valve. In both valves it is of essentially 

 the same size and ovate-triangular in outline, the apex of the 

 triangle pointing forward and usually surrounded by a con- 

 spicuous callosity. 



The ventral (?) valve bears two well defined central adductors 

 occupying the same relative position as in Crania; these 

 impressions are usually simple, but appear to be sometimes com- 

 plicated 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 margin. 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. 



Distribution, Lower Silurian to Lower Carboniferous. 



131 



