408 THE PALEONTOLOGY OF MINNESOTA. 



[Triplecia. 



Formation and locality. Common in the Hudson River group at, and two miles east of, Spring 

 Valley, Minnesota. Rather rare in the middle and upper portion of the same formation in Ohio and 

 Indiana. 



Collectors. W. H. Scofleld, E. O. Ulrich and the writers. Also in the collection of Dr. C. H. 

 Robbing, Wykoff, Minnesota. 



Mus. Reg. Nos. 4098, 8155. 



Genus TRIPLECIA, Hall. 



1858. Trip'esia, HALL. Twelfth Report New York State Cabinet of Natural History, p. 44, figs. 1-3. 

 1892. Tripleeia, HALL. Palaeontology of New York, vol. viii, pt. i, p. 269. 



Description: " Shell trilobate, transverse, unequally biconvex. Hinge-line straight 

 and quite short. Pedicle valve shallow, convex about the beak, but depressed anter- 

 iorly by a broad and deep median sinus; cardinal area low, erect and well defined; 

 delthyrium covered by a narrow convex [or flat] plate, with a circular foramen at 

 the apex. In the interior the teeth are well developed and supported by short dental 

 lamellae longitudinally dividing the umbonal cavity near the apex. Muscular area 

 small, comprising two lateral scars, separated by a longer central adductor impres- 

 sion. The brachial valve is very convex and bears a strong median fold. The car- 

 dinal area is very narrow and the beak closely incurved. In the interior is an erect 

 cardinal process, which is deeply bifurcated, the distal extremity of each branch 

 bearing a single deep groove. This process is supported on a subrostral callosity, 

 which also bears two short spiniform crural points at its base. Shell substance 

 fibrous, impunctate(?). Surface with obscure concentric growth lines, and fine 

 radiating striae on the inner laminae; in rare instances there are radiating lines 

 on the exterior. 



"Type: Atrypa extans Emmons." (Hall, 1892, op. cit.) 



Plicated species of this genus are unusual, Triplecia being characterized mainly 

 by smooth forms. T. radiata Whitfield and the following new species are the only 

 ones known in America. In Britain there is but one, Triplecia sporiferoides McCoy, 

 sp., from the Upper Llandeilo and Caradoc, and another, T. cava Barrande, sp., from 

 Etage D 2 of Bohemia. One is unwilling at first to regard these plicated species as 

 congeneric with the smooth forms, but upon examination it is seen that the generic 

 characters common to the one section are also present in the other. They were 

 derived from smooth forms, since all of the nepionic and early neologic growth is 

 .without a trace of plications, they being first introduced during the later neologic 

 stages. 



Species of Triplecia are known from the Calciferous to the Upper Silurian. 



