598 THE UPPER SILURIAN. 



This species differs from the R.formosa of the Lower Helderberg 

 rocks of New York in the plications being more slender, in the more 

 defined sinus of the ventral valve, and the continuation of the two 

 small folds in the sinus nearly to the Leak. Arisaig, coll. J. W. D. 



Rhynchonella Saffbrdi. Shell varying in form from ovoid to globose. 

 Full grown specimens usually wider than long, and sometimes becom- 

 ing extremely ventricose, so that the diameter across the two valves 

 much exceeds the length. Ventral valve depressed convex, with the 

 beak minute, closely incurved ; dorsal valve very ventricose, most 

 prominent toward the front. Cardinal slope a little depressed, sides 

 rounded, and the front in direct line flattened but not depressed. 

 Surface finely plicated, plications little elevated, rounded or scarcely 

 subangular, about five or six depressed in the flattened sinus of the 

 ventral valve, and a corresponding number raised on the flattened 

 mesial elevation, which rises abruptly, though usually but slightly 

 above the lateral portions of the shell. From ten to fourteen plications 

 mark the surface on each side of the mesial fold and sinus. Plications 

 in front marked by a sharp groove along the centre, and those of each 

 valve deeply interlocking. 



This species resembles the R. nucleolata of the Lower Helderberg 

 rocks of New York, and in some specimens it approaches to R. 

 ventricosa, but is always much more finely plicated than either. 

 It closely resembles the R. Wilsoni of Europe in its general form, 

 but the plications are more rounded and somewhat coarser, and 

 while in that species the sinus causes no depression in the ventral 

 valve below the general surface of the shell, in ours there is an 

 abrupt depression as well as a slightly abrupt elevation on the dorsal 

 valve, while there is no similar feature in the R. Wilsoni* 



The Nova Scotia specimens are in all respects identical with those 

 from Tennessee. Arisaig, Earlton, coll. J. W. D. 



The geological position of the specimen from Tennessee is in 

 rocks of the age of the Lower Helderberg group, associated with 

 Pentamerus galeatus, P. Verneuili, Spirifer macropleura, Spirifcr 

 perlamellosa, Spirifer cycloptera, and others. 



Rhynchonella equiradiata, Hall. Arisaig, coll. J. W. D. 



Rhynchonella neglecta^ Hall. Arisaig, coll. J. AV. D. 



Rhynchonella, n. s. (?) allied to R. transversa. Arisaig, coll. J. W. D. 



Leptocelia intermedia, Hall (Fig. 202). Shell concavo-convex; 

 outline semi-elliptical, cardinal extremities rounded, and the hinge- 

 line a little shorter than the greatest width of the shell ; ventral 



* Sowerby, M. C, vol. ii., page 38, says: The " sinus at the front, although deep, 

 does not altei- the evenness of the sui-face." 



