376 REPORT ON THE STATE CABINET. 



the}' belonged to the Genus Obolus. There are certain points of differ- 

 ence, however, which I have been inclined to believe are of generic 

 importance, and I am not entirely satisfied in referring them to that 

 genus, as illustrated, though closely allied to it. 



In the mean time, Mr. Billings has published a Genus Trimerella, 

 illustrating it by a specimen from the Guelph limestone. Now the fossils 

 under consideration, in the earlier stages of growth, have the transverse 

 plate but partially attached at the sides ; and the processes shown in Mr. 

 Billings' figures are sometimes slightly simulated by the casts of the 

 spaces on each side of the median septum, which extend between this 

 transverse plate and the exterior shell. There are, however, no evidences 

 of three longitudinal septa. 



The exterior of the shell is unknown, but probably is essentially 

 smooth, or with only lines of growth. From the fact that in numerous 

 specimens collected from these rocks there is no shell preserved, I infer 

 that it was calcareous and not phosphatic as in Lingula and Discina, 

 which usually preserve the shell in all the dolomitic limestones. 



Formation and Localitij. — In the Leclaire limestone, upper part of the 

 Niagara group, at Leclaire, Iowa, and in limestone of the same age at 

 Racine, Wisconsin. 



■') 



GENUS STROPHODONTA, Hall. 

 Strophodonta profunda. Hall. 



PLATE XIII, FIGS. 3, i. 



Leptmna profunda, Hall. Palasontology N. Y., II, p. 61, Plate xsi, figs. 4, 5. 1852. 

 StrophomcTia niagarensis, W. & M. ; in Mem. Bost. Soc. jSTat. Hist., I, p. 92, Plate ii, flg. 9. 1865. 



Shell large, the full grown individuals having a width of more than two 

 inches, with a length exceeding one inch and a half ; deeply con- 

 cavo-convex; the extremities sometimes angular or extended, but 

 more often in the casts obtuse or rounded. Surface of young shells 

 somewhat regularly marked by strong elevated striae, with four, 

 five or more finer stri^ between. In older shells the striae become 

 more irregularly fasciculate, and the stronger ones rise in unequal 

 ridges upon the surface. This feature is, in greater or less degree, 

 impressed upon the casts of the interior, which, combined with 

 strong vascular marldngs, gives a distinguishing character to speci- 

 mens in that condition. The hinge line is crenulate, with a broad 

 foramen ; the muscular impression of the ventral valve is ovate or 

 flabelliform, and extends for two-fifths the length of the shell. 



