GENUS STROPIIODONTA. 369 



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 there is no shell preserved, I infer that it was calcareous and not 

 phosphatic as in Lingula and Discina. 



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

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

 Racine, Wisconsin. 



STROPHODONTA PROFUNDA. 



PLATE 13 (4), FIGS. 3 & 4. 



LeptcBna profunda : Hall, Pal. N. Y., Yol. ii, p. 61, plate xxi, fi<];s. 4 and 5. 1852. 

 Sirophomena niagarensis : AVinchell &c Marct, Memoirs of the Boston Society of 

 Natural History, Yol. i, page U2, plate ii,fig. 9. 



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

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

 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 striae 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 markings, gives a distinguishing character to specimens 

 in that condition. 

 The hinge-line is crenulate, with a broad foramen; the muscular impres- 

 sion of the ventral valve is ovate or flabelliform, and extends for two- 

 fifths the length of the shell. There is a central longitudinal callosity 

 extending from the apex sometimes for one-third of the length of the 

 muscular impression. 



The Sirophomena niagarensis of Winchell & Marcy (loc. cit.), appears 

 to me to be identical with this one. The specimens in my possession, from 

 western localities, show a considerable variety of form and proportions, 

 and yet preserving the essential characters of the species. 



A specimen communicated by Prof. Winchell under the name -S. 

 niagarejisis, has the divaricator scars proportionally a little more elongate 

 than usual, but the same feature is preserved in other specimens. 



The specimens figured are a young or medium form, preserving a part of 

 the exterior surface of the ventral valve on the lower margin, while the 

 other portion is the impression of the exterior of the dorsal valve. Fig. 4 

 is the cast of the interior of the ventral valve. 



Formation and locality. In limestone of Niagara age, at Racine (Wis- 

 consin), Bridgeport (Illinois), and Waldron (Indiana). 



[Asisem. No. 239.J 47 



