124 Cephalopoda of the Cincinnati Group. 



Silurian rocks of America. Some specimens have been found in 

 a fragmentary condition, that appear to belong to the family 

 (rp'oceraticke, and others that poi-sibly belong to Fhragmoceratidce. 

 But I have concluded, because of the doubt about the genera of these 

 latter specimens, to confine this monograph to the fir.st two families 

 above mentioned. 



Family Orthoceratida;. 



Shell externa], straight, conical ; divided by numerous septa into 

 closed chambers ; last chamber much the largest, ibr habitation ; 

 siphuncle piercing all the chambers, varying in position, from the 

 center to the outer margin, and varying in size and form in the dif- 

 ferent species. 



This family includes the genera Orthoceras, Endoceras, and Ormoceras. 



Genus Orthoceras — (Brkyn). 



[Etym.— Orthos, straight; ke?-as, horo.] 



Shell conical, straight ; greater part of the posterior end traversed 

 by convex, transverse septa; transverse section circular, oval, or more 

 or less triangular; siphuncle cylindrical, or dilated between the 

 chambers, and varying in position from the center to the outer margin; 

 surface smooth, longitudinally or transversely lined. 



Orthoceras Mohri — (S. A. Miller). 



Fig. 10.— Orthoceras Mohri. Longitudinal section showing siphuncle. 



Shell elongated, tapering very regularly, at the rate of about 0.16 

 inch to the inch, to an acute point. Septa rather strongly arched, and 

 distant, about one fourth the diameter of the shell. (Measurement 

 of three different polished specimens produced the following result: 

 diameter 0.62 inch, septa distant, 0,15 inch ; diameter 0.50 inch, 

 .septa distant 0.12 inch ; diameter 0.25 inch, septa distant 0.06 inch.) 

 Siphuncle central, and having the appearance of a connected series of 

 oval beads, with the larger ends directed forward, and gradually 



