INVERTEBRATES. 503 



socket processes are seen, by their impressions in the cast, to be small- 

 not united, and scarcely assuming the character of plates ; while the 

 crural processes extended from their inner lower sides forward nearly 

 parallel, so as to leave two slender, deep perforations in the cast. The 

 surface of the young shell appears to have been smooth, or only marked 

 with concentric strise, but internal casts of large individuals sometimes 

 show very faint traces of a few broad, irregular, radiating flattened 

 ridges. 



It is probable that this species is most nearly allied to StricMandinia 

 Davidsoni, of Billings (Geol. Mag., Vol. V, PL IV, fig. 1, la), which in 

 some stages of its growth it resembled rather nearly in form. In all 

 the large examples, however, it differs extremely from that shell in its 

 remarkable narrowness across the umboues, and its truncated or flat- 

 tened posterior lateral margins. Its front is also less produced and less 

 narrowly rounded in the middle in these larger specimens. 



Position and locality : All the specimens of this species we have seen 

 were found loose in Carroll county, Illinois, near rocks of the age of the 

 Niagara Group. They are all in the condition of white quartz casts of 

 the interior. 



CEPHALOPODA. 



Genus OETHOCERAS, Auct. 

 Obtiiocebas cbebristbiatum, M. and W. 



PI. 26, Fig. 2. 

 Orthocerai crebruitrmtum, Meek and Worthen, 1865. Proceed. Philad. Acad. Nat. Sci., p. 255. 



Shell attaining a medium size, rather rapidly tapering, compressed, 

 (in part probably due to accidental pressure) ; section elliptical ; septa 

 transverse, rather deeply concave, distant less than one-third the 

 greater diameter of the shell at the point of measurement; siphon 

 apparently subcentral. Surface ornamented with numerous closely and 

 very regularly arranged, equal, thread-like annular striae, of the same 

 breadth as the depressions between, and differing but slightly in size 

 throughout the entire length of the shell. 



Length of the typical specimen (which is partly septate and imperfect 

 at both extremities) 12.50 inches, greater diameter at larger end 4.20 

 inches, smaller diameter of same 2.56 inches. Greater diameter of 

 the smaller end about 2.13 inches, smaller diameter of same 1.08 inch. 



