^24 THE PALEONTOLOGY OF MINNESOTA. 



[Sphenollum parallemm. 



have evidence to show that this disproportionate development of the posterior end 

 was a gradual process, an undescribed species from the TJtica horizon at Cincinnati 

 being intermediate in this respect between the Trenton forms and those occurring 

 in the middle and upper beds of the Cincinnati group. 



Sphenolitjm paeallelum, n. sp. 



PLATE XXXVI, FIGS. 42 and 43. 



Shell elongate subovate, rather strongly convex, the thickness, hight and length 

 respectively as one is to one and two and one- fourth. Dorsal margin straight, nearly 

 parallel with the ventral, terminating posteriorly in an obtuse angle where it joins 

 the obliquely rounded posterior margin; anterior end short, apparently narrowly 

 rounded; basal line very gently convex; posterior end rather abruptly rounded in 

 the lower half. Beaks prominent, full, incurved; umbonal ridge strongly convex, 

 somewhat emphasized by a slight furrow immediately above it in the cardinal slope; 

 another obscure furrow borders the dorsal edge. A small but well marked lanule 

 in front of the beaks, and a narrow and rather illy defined channel behind them. 

 Central and anterior parts of valves rather strongly convex. Surface marked con- 

 centrically with very fine striae and a few more or less obscure undulations. The 

 latter are more distinct and regular on the umbonal ridge than elsewhere. 



The subparallel margins distinguish this species from the more typical forms of 

 the species described by Miller from the Cincinnati rocks. 



Formation and looality.—" Lower Blue" beds of the Trenton formation, Mineral Point, -Wisconsin. 

 Mus. Beg, No. 8346. 



Sphenolium stbiatitm, n. sp. 



PLATE XXXVI, FIGS. 44 and 45. 



The shape and general expression of this shell is almost exactly the same as in 

 the preceding, S..parallelum, yet when critically compared certain differences are 

 observed which render a separation necessary. The specimens are not very perfect 

 casts of the interior and exterior, still they preserve traces of very fine radiating 

 lines on the umbonal ridge and a few coarser ones on the cardinal slope which, if 

 such had been present on 8. parallelum, would undoubtedly show on the excellently 

 preserved cast upon which that species is founded. The Galena specimens again 

 present a number of small, regular and short concentric folds on the anterior end, 

 but they are wanting on the sides and posterior end where the folds are rather 

 distinct in the Lower Trenton species. In comparing the outlines a slight diffierence 

 is to be detected in the postero-cardinal region where, instead of being subangular 



