376 THE PALEONTOLOGY OF MINNESOTA. 



[Pholidops. 



Genus PHOLIDOPS, Hall. 



1859. Pholidops, HALL. Palaeontology of New York, vol. lii, pp. 489, 490. 

 1892. Pholidops, HALL. Ibidem, vol. viii, pt. i, p. 155. 



Description: "Shells small, patelliform, equivalve, equiconvex, inarticulate, 

 unattached. Outline oval or subelliptical; apex su'bcentral, excentric or marginal, 

 sometimes terminal and produced. Surface marked by strong, concentric, often 

 lamellose lines of growth, which are crowded on the posterior and distant on the 

 anterior portion of the valves; these are sometimes crossed by faint, interrupted 

 radiating lines. In the interior the surfaces of contact make a broad, smooth, flat 

 or slightly convex border, somewhat 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 it is of essentially the same size and subtriangular 

 in outline, the apex of the triangle pointing forward and usually surrounded by a 

 conspicuous 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 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 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 cal- 

 careous and impunctate(?)." (Hall, 1892, op. cit.) 



Type: Orbicula ? squamiformis Hall. 



PHOLIDOPS TRENTONENSIS Hall, var. MINOR, n. var. 



PLATE XXIX, FIG. 40. 



The original description of P. trentonensis Hall* is as follows: "Shell small, 

 broadly oval, very depressed-convex. Apex situated near the cardinal extremity. 

 Surface marked by strong, concentric, lamellose lines of growth." 



The Minnesota specimens in outline, convexity of valves, position of the apex 

 and the strong lamellose lines of growth, agree with these parts as described and 

 illustrated in P. trentonensis Hall. They are, however, one-half, but more often less 



* Descript. of new species of Orinoidea and other Fossils, p. 14, 1866. Twenty-fourth Rep. N. Y. State Cab. Nat. Hist 

 p. 221, pi. TII. fig. 8, 1872. 



