88 



chamber of habitation is two inches in depth, and the greatest diameter 

 two inches. The aperture is often very much constricted. 



1, — Chamber of habitation only, length 2 inches ; diameter of aperture, 

 16 lines ; dorso ventral diameter at last septum, 22 lines ; lateral 

 diameter, 24 lines. 



2, — Chamber of habitation and last two septa ; dorso — ventral 

 diameter of aperture, 17 lines ; lateral, 18 lines ; dorso ventral diameter 

 at the last septum, 22 lines ; lateral, 24 lines : depth of chamber of habi- 

 tation, 26 lines. 



3. — Length, 4 inches, but, when perfect, probably 6 inches ; aperture 

 visible on the dorsal side only ; length of chamber of habitation, 30 lines ; 

 section at last septum nearly circular, and 25 lines across ; there are 7 

 septa in 20 lines on the dorsal side. On the ventral side there are 

 about 3 septa to the inch in large specimens. 



This species differs from 0. Pettiti in being more slender, and in hav- 

 ing the ventricose position more extended in length in proportion to the whole 

 length of the shell. The surface is covered with fine obscure transverse 

 striaa, which make a sinus on the median line of the ventral aspect. Faint 

 longitudinal sulci are visible on the cast. 



Grimsby ; Niagara formation. J. Pettit. 



Genus, Streptoceras, N. G. 



The above generic name is proposed for species having the form of 

 Oneoceras, but with a tri-lobed aperture like Pkragmoceras. 



S. Janus, n. sp. — Large individuals are seven inches in length, and 

 two and a half inches in greatest diameter ; gradually enlarging from 

 the aperture to about the mid-length ; then more abruptly contracting ; 

 the apical fourth of the length more slowly diminishing. Section at 

 the aperture sub-triangular ; in the main body of the sLiell broad ovate or 

 nearly circular and towards the apex circular, less com ex on the dorsal 

 than on the ventral aspect. Aperture, in contour, a triangle with the 

 angles rounded, forming three lobes, one of which is ventral, and the other 

 two lateral, but near the dorsal aspect. The ventral lobe is narrowly 

 rounded, and forms a projection hke the lip of a pitcher. The lateral 

 edges (of the aperture) behind the ventral lobe are at first gently concave, 

 and then gently convex, gradually rounding into the two lateral lobes ; the 

 dorsal edge between the two lateral lobes, is gently concave. The outline 

 of the body of the shell on the dorsal aspect is nearly straight or slightly 

 concave from the aperture for two-thirds the length, then curved. The 

 ventral aspect almost uniformly arched to a radius of about four inches in 

 a specimen seven inches in length. Siphuncle situated in the median line of 

 the ventral aspect, moniliform, the segments nearly three lines in diameter. 

 Septa about four to the inch on the side, becoming more numerous towards 



