372 THE PALEONTOLOGY OF MINNESOTA. 



[Gasteropegmata. 



Suborder GASTEROPEGMATA, Waagen. 



Family CRANIID^], King. 

 Genus CRANIA, Retzius. 



1781. Crania, RETZIUS. Schriften der Berliner Gesellsch. Naturf. Freunden, vol. ii, p. 72. 

 1892. Crania, HALL. Palaeontology of New York, vol. viii, pt. i, p. 145. 



Description: " Shell inequivalve, inarticulated, without perforation for a pedicle; 

 subcircular in outline, generally somewhat transverse across the posterior margin; 

 attached by the apex or the entire surface of the lower valve. Ventral or lower 

 valve depressed-conical or conforming to the surface to which it is attached. Dorsal 

 or upper valve more or less conical, with a subcentral, posteriorly-directed apex. 

 External surface of the valves usually smooth, sometimes spinose or with concentric 

 or radiating striae. In the interior of both valves are two pairs of large adductor 

 scars, the posterior of which are close upon the margin and widely separated, the 

 anterior near the center of the shell and close together, more approximate in the 

 lower than in the upper valve. These posterior scars are often strongly elevated on 

 a central callosity which surrounds their anterior margins. The margin of the lower 

 valve is usually broad and thickened. Impressions of the pallial genital canals 

 coarsely digitate. 



"Shell substance calcareous; strongly punctated by vertical canals which become 

 subdivided toward the epidermal surface. 



" Type : Crania craniolaris Linne." (Hall, op. cit.) 



This genus had its origin in the Trenton formation and thence has existed 

 through all geological time up to the present. .One species is reported from the 

 Chazy and another from the middle Primordial, neither of which have furnished 

 undoubted evidence of their belonging to this genus. 



CKANIA SETIGERA Hall. 



PLATE XXIX, PIGS. 32 and 33. 



1866. Crania setigera HALL. Description of new species of Crinoidea and other Fossils, p. 12. 

 1872. Crania setigera HALL. Twenty-fourth Rep. N. Y. State Cab. Nat. Hist., p. 220, pi. vn, flg. 15. 

 1892. Crania setigera HALL. Paleontology of New York, vol. viii, pt. i, pi. ivH, flgs. 14-16. 



Original description: "Shell small, suborbicular; length greater than width; 

 cardinal margin nearly straight. Dorsal valve convex; beak elevated, pointed, situ- 

 ated nearly one-third the length of the valve from the cardinal border. 



"Surface marked by comparatively coarse pustules or setas, which are more 

 distant [distinct] near the margin of the shell." 



The individuals of this species from the thin-bedded Trenton limestone have the 

 surface pustules strongly elevated, appearing more like short spines. Those from 



