87 



at first somewhat abruptly, and then gradually. In the smaller specimens 

 the section is nearly circular, but in the larger it is more or less ovate, the 

 dorsal, or side of the concave curve, being less convex than the ventral. 

 The siphuncle is situated in the median line, near, but not in contact with 

 the shell, about a line in diameter at the passage, but inflated to two or 

 three lines between the septa. The chamber of habitation is large, ap- 

 parently one-third the whole bulk of the shell There are about fom* septa 

 to the inch measured on the side of a specimen five inches in length. The 

 surface is marked with obscure engirdling striee which make a slight bend 

 towards the apex along the median line of the ventral aspect. There are 

 also faint indications of longitudinal sulci. The following are the dimen- 

 sions of three specimens. 



1. — Length following the curve along the middle of the ventral side 5 

 inches ; dorso-ventral diameter of the aperture 18 lines ; lateral diameter 

 22 lines ; dorso-ventral diameter at the first septum 24 lines ; lateral 

 diameter 28 lines ; diameter at the twelfth septum (at which point the 

 section is circular) 10 lines ; depth of chamber of habitation 22 lines. The 

 ventral outline is curved to a radius of about 3^ inches. For about 9 

 lines from the aperture the shell enlarges very gradually, and then sud- 

 denly expands two or three lines. 



2. — Length 4| inches on the ventral curve ; dorso-ventral diameter at 

 the aperture 17 lines ; lateral diameter 21 lines ; dorso-ventral diameter 

 at the last septum 22 lines ; lateral diameter 27 lines ; depth of chamber 

 of habitation 24 lines ; ventral aspect curved to a radius of 4 inches. The 

 smaller extremity, where broken oflF, is about 9 lines in diameter. 



3. — Length 3i inches on the ventral curve ; dorso ventral diameter at 

 the aperture 14 lines ; lateral diameter 16 lines ; dorso-ventral diameter at 

 the last septum 19 lines ; lateral diameter 22 lines ; curved to a radius of 

 about 3 inches. 



In all the specimens the dorsal aspect is less convex than the ventral 

 but in some the difference is very slight. At first sight, taking extreme 

 forms, it might well be thought that there are several species ; but there 

 is a transition, and I do not see how they can be separated. The indivi- 

 duals are numerous, but mostly in fragments. Grimsby ; Niagara for- 

 mation. This species is dedicated to the discoverer, Johnson Pettit, Esq., 

 of Grimsby C. W., who has done good service to science in collecting 

 many fossils from a formation difficult to work out. 



0. Thales, n. sp. — Length five or six inches, proportionately not so 

 ventricose as 0. Pettiti ; siphuncle not in the median line, but a little to 

 the right thereof, moniliform, the segments two lines in diameter, nearly 

 in contact with the shell ; about four septa to the inch on the side. In a 

 a specimen which, when perfect, must have been six inches in length, the 



