368 THE PALEONTOLOGY OF MINNESOTA. 



LTrcinatis liuronensis. 



" Shell-substance composed of an outer calcareous layer with a series of inner 

 corneous lamellae. The outer layer varies in thickness in different species and is 

 coarsely punctated by the pittings constituting the surface ornamentation. The 

 corneous layers are impunctate." (Hall, op. cit.) 



Type : Trematis terminalis Sharpe (not Emmons)=T. millepundata Hall. 



TEEMATIS HURONENSIS Billings? 



FIG. 29. Billings' original figure of T. huronensis. a, lower valve, b, longitudinal section, showing 

 the curvature of both valves; c, a portion of the surface enlarged. 



1862. Trematis huronensis BILLINGS. Palaeozoic Fossils, vol. i, p. 53, figs. 59a, 596, 59c. 



1863. Trematis huronenis BILLINGS. Geology of Canada, p. 159, figs. 130a, 130e. 



1892. Productella minneapolis SABDESON. Bulletin of the Minnesota Academy of Natural Sciences, 



vol. iii, p. 332, pi. iv, flgs. 11, 12. 



Of Trematis we have seen but four imperfect specimens. Since they preserve only 

 the dorsal valve, no direct comparison can be made with the ventral valve of T. huron- 

 ensis Billings, and therefore this identification is provisional. The surface pittings 

 in these specimens are similar to those in T. huronensis, i. e, they are arranged in 

 radiating lines, not in quincunx, and in this respect differ from T. terminalis Emmons, 

 T. millepundata Hall, and T. quincuncialis Miller and Dyer. Our specimens vary 

 somewhat from Billings' species, in that the radial surface depressions become 

 obsolete towards the anterior margin. 



The " ventral valves " of Productella minneapolis Sardeson are identical with the 

 upper or dorsal valve of the shells identified by the writers with Trematis huronensis. 

 Moreover, Productella is not known below the Devonian. 



Formation and locality. From the Trenton .limestone in the quarries along the river bluffs, and 

 near the base of the Trenton shales at Minneapolis, Minnesota. The Canadian specimens are from the 

 Black River limestone of Pallideau islands, lake Huron. 



Collectors. C. L. Herrick and C. Schuchert. 

 Mus. Reg. No. 7691. 



