382 THE PALEONTOLOGY OF MINNESOTA. 



[Pentameridas. 



line is longer, and the cardinal area of the ventral valve is wider, as compared with 

 that species, while a more conspicuous mesial sinus and fold are present in S. anthon- 

 ensis. Further in the apical portion of the delthyrium there is a small spondylium, 

 and in the dorsal valve the space between the crural plates is thickened by shell 

 deposit and is medially divided by a sharp, but low, cardinal process, the whole being 

 drawn out into a long, angular, medial septum. 



Formation and locality. Bare in the Trenton limestone and shales at Minneapolis, St. Paul and 

 near Cannon Falls, Minnesota. In the Trenton at Dixon, Illinois. Common in the "Glade limestone'' 

 at Lebanon, Tennessee. 



Collectors. E. O. Ulrich, W. H. Scofleld and C. Schuchert. 

 Mus. Reg. No. 8252. 



Family PENTAMERID^E, McCoy. 



Genus ANASTROPHIA, Hall. 

 ANASTROPHIA ? HEMIPLICATA Hall, sp. 



PLATE XXX, FIGS. 29-31. 



1847. Atrypa hemiplicata HALL. Palaeontology of New York, vol. i, p. 144, pi. xxxm, flg. 10. 



1856. Atrypa hemiplicata BILLINGS. Canadian Naturalist and Geologist, vol. i, p. 208, flgs. 20-23. 



1859. Pentamerus hemiplicatus BILLINGS. Canadian Journal, vol. iv, p. 316. 



1859. Pentamerus hemiplicatus HALL. Twelfth Report N. Y. State Cabinet of Natural History, p. 66 



1863. Camarella hemiplicata BILLINGS. Geology of Canada, p. 168, flg. 154. 



1892. Camarella bernensis SAUDESON. Bulletin of the Minnesota Academy of Natural Sciences, 



vol. iii, p. 328, pi. iv, flgs. 4-6. 



Original description: "Subglobose, pentagonal, wider than long, thickness often 

 equal to the length; cardinal line distinct, short, with (in some specimens) the 

 appearance of a small area on the dorsal [ventral] valve; dorsal [ventral] valve 

 depressed-convex, with an abrupt, broad, not deep sinus, which commences nearly 

 half way from the beak to the base, the beak very small and closely incurved [with 

 a small triangular delthyrium underneath]; ventral [dorsal] valve very convex, 

 becoming gibbous with a broad mesial elevation, commencing one-third of the dis- 

 tance from beak to base, more gibbous towards the beak; sinus marked by two or 

 three strong plications, with three or four upon the mesial lobe, and two or three 

 on each side, all of which reach from one third to one-half the distance from 

 the base to the beak of the shell, leaving the upper half entirely free from these 

 markings; entire surface ornamented by fine, concentric, filiform, subimbrbating 

 lines, which are more conspicuous' towards the base of the shell and beautifully 

 undulated in crossing the plications." 



In Minnesota this species occurs not uncommonly near the base of the Galena 

 and differs from New York examples in having the umbo of the dorsal valve more 

 tumid and elevated beyond that of the ventral valve. The transverse diameter in 

 the former is also shorter, while the individuals are commonly smaller than thjse 

 from eastern localities. 



