NATURAL SCIENCES OF PHILADELPHIA. 255 



larger size, will also distinguish it from T. incurvus, of Shumard, (Missouri 

 Report, pi. B, fig. 6a, b.) 



Locality and position. Cincinnati Group, of Lower Silurian ; Oswego, Ken- 



dall Co., Ill 







Tentaculites Sterlingensis, M. & W. 



Comp. T.flexuosus, Hall. Palaeont. N. Y., i. p. 2S4, (not ib, 92.) 



Shell small, slightly arched, and gradually tapering to a point ; section cir- 

 cular ; annulations prominent, angular, rising abruptly from the surface, usu- 

 ally about their own breadth apart ; constrictions between the annulations 

 with fine, sharply elevated, longitudinal striae, which are not continued upon 

 the rings. 



Lengtu, 056 inch ; breadth at the larger end, 0-08 inch ; annulations live 

 in the space of of an inch at the larger end, and nine or ten in the same 

 6pace at the smaller end. Longitudinal striae, five in the space of 0*02 inch. 



It is not improbable that this will prove to be the form from the so-called 

 Hudson River group, referred by Prof. Hall to his T. Jlexuosus, in vol. i. 

 p. 284, Palaeont. N. Y. As that specific name, however, was founded upon 

 a Trenton fossil, described as being septate, and having nine rings in of an 

 inch, (being,as is now supposed, the column of a Cystidian), the name /eiu- 

 osus could not be properly applied to this form, even if identical with the 

 New York species from the higher position. 



It will be distinguished from T. incurvus, of Shumard, from the Cape Girar- 

 deau Limestone, which it resembles in size and form, by having its annula- 

 lations arranged about their own breadth, instead of twice that distance apart, 

 as well as in having the longitudinal stria? only defined between the rings, 

 instead of also upon them. 



It seems to be very closely allied to T. distans, Hall, of the Clinton Group, 

 but differs in being curved instead of straight, as well as in being less rapidly 

 expanding towards the larger end. 



From the last of the two foregoing species it will be readily distinguished 

 by its much smaller size, more sharply elevated rings, and distinct longitudi- 

 nal striae. 



Locality and position. Sterling, Illinois. Cincinnati Group, of the Lower 

 Silurian series. 



CEPHALOPODA. 



Genus ORTHOCERAS, Auct. 

 Orthoceras crebristkiatum, M. & W. 



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

 part probably due to accidental pressure) ; section ellipitical ; 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 dif- 

 fering 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 the larger end, 4*20 inches ; 

 smaller do. of same, 2-56 inches. Greater diameter of the smaller end, about 

 2 - 13 inches ; smaller do. of same, l'OS inches. Angle of divergence, measuring 

 along the narrower sides, 11. Annular striae, 8 in 0*20 inch at the larger end, 

 and 9 or 10 in the same space at the smaller end. 



The most marked character of this species is its very regularly arranged, 

 equal striae, which seem to pass almost, if not quite, directly around the shell. 

 They appear to be simple, uninterrupted and everywhere arranged their own 

 breadth apart. It diners from O. Lqphami, from the same rock, in its much 



1865.] 



