FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 135 



ter to the surface, and where the shell is exfoliated the plications are nodose. 

 In some specimens, distinct, fine radiating striae can be observed, but this is 

 of rare occurrence. There is often a retrorse curving of the concentric striae 

 in the center of the mesial sinus, and sometimes a slight elevation on that line. 

 (Hall.) 



Prof. Hall, who first described this species, compares it with Spirifera euru- 

 teines of Owen. In regard to size, the euruteines is almost twice as large as 

 the varicosa, at least we find plenty specimens of the former species which 

 have double the width of the latter one. Spir. varicosa is always more trans- 

 verse, that is, the excess of its width over its length is considerably greater 

 than the excess of the width over the length in Spir. euruteines. In Spir. 

 varicosa the plications are more angular, and the concentric lines of growth 

 greatly stronger than in Spir. euruteines. It differs from the latter also by its 

 abrupt and angular mesial fold. This species shows very little variations in 



the more than one hundred specimens before me. 



/ 



Formation and Locality. Found in great abundance, and in most perfectly preserved specimens, 

 in the Corniferous limestone around the Falls of the Ohio, in Kentucky and Indiana. 



Genus Trematospira. Han. 



Trematospira, Hall. Pal. N. Y., Vol. 3, page 2071859. 



Etymology : trema, a foramen; spira, a spire; alluding to the perforation in the beak of the ventral valve. 



Shell transverse, elliptical or sub-rhomboidal, furnished with internal spires 

 (arranged as in Spirifera) ; hinge-line shorter than width of shell ; cardinal 

 angles rounded. Valves articulated by teeth and sockets ; beak of ventral 

 valve produced or incurved, and truncated by a small, round perforation, 

 separated from the hinge-line by a deltidium. A deep triangular pit or fora- 

 men, beneath the beak of ventral valve, which is filled by beak of the dorsal 

 valve. False area sometimes defined. Surface marked either with strong, 

 simple plications or finer fasciculate or bifurcating striae, which cover also 

 the mesial elevation and depression. Shell structure punctate. (Hall.) 



In the extension of the hinge-line, the mesial sinus, the internal spires, and, 

 partly, in the exterior markings, this genus resembles Spirifera and Spiriferina. 

 In the perforate beak, false area and incurving of the dorsal beak beneath 

 that of the other valve, it resembles Atrypa, while one species has the gen- 

 eral aspect of Rhynchonella. The broad triangular foramen for the recep- 

 tion of the dorsal beak is a constant and conspicuous feature of this genus. 



