Hyatt.] 282 [April 4, 



Oncoceratidae. 



This family includes forms with peculiarly attenuated apices, or 

 young, the whorl increasing in the adult stages very rapidly in 

 size. The living chambers are generally short, and constricted 

 above, but the apertures are open. Siphon is near the venter, 

 and generally, there is a ventral lobe. The young have straight 

 sutures and these are retained in some adults. 



Eremoceras, 1 nobis, includes arcuate Silurian species with open 

 apertures, short living chambers, whorl in section elliptical; 

 sutures with dorsal saddles, almost straight lateral sutures and 

 ventral lobes. Type, Erem. (Cyrt.) Syphax, sp. Bill. Pal. Foss., 

 Vol. 1, p. 194, fig. 178, Mus. Geol. Surv. Can. 



Clinoceras, Mascke, Zeit. d. Deutsch. Geol. Gesell., Vol. 28, 

 p. 49, pi. 1, includes sj3ecies similar to Oncoceras but with siphon 

 between the centre, and dorsum; living chamber and aperture 

 similar, but not swollen, and form very slightly arcuate ; sutures 

 with a minute annular lobe according to Mascke. Ciin. (Cyr). 

 exiguum, sp. Bill. Can. Nat. Vol. 5, p. 172, and One. mumiaforme 

 Whit. Geol. Wise, Vol. 4, 1873-79, pi. 7, have straight sutures and 

 are closely allied to type of Clinoceras in form. 



Oncoceras, Hall, Nat. Hist. N". Y., Vol. 1, p. 197 is similar in 

 form to Clinoceras, but the whorl is more depressed, and the 

 ventral aspect fusiform. It has sutures similar to Eremoceras, 

 but the ventral lobe is often narrow, and pointed. The lateral 

 sutures are deeper, and in some species there are dorsal lobes- 

 Siphon is ventral. The living chamber is constricted near the 

 aperture, and much dilated below. One. (Cyrt.) heteroclitum. 

 Barr. Syst. Sil., pi. 118, 475, has the peculiar swollen living 

 chamber of Oncoceras, and ventral siphon. J Am. Mus. N". Y. 



Hercoceratidae. 



This family has, in the normal forms, trapezoidal whorls with 

 abdomen broader than venter, one line of large tubercles along 

 the edge of abdomen, and sutures with ventral and dorsal lobes. 

 Shells not costated. 



Ptyssoceras, 2 nobis, includes Silurian arcuate shells with 



i "EpT]|ios, alone. 

 * IItvW<i>, fold. 



