30 EOCENE MOLLUSCA. 
This species presents considerable variations in the form, as well of the callus as 
of the rostrum; and, in fact, M. Deshayes has separated it into the three species 
B. longispina, B. longirostris, and B. Blainvillii, chiefly on account of the different 
conditions of the rostrum. M. d’Orbigny, attributing the variableness of the rostrum 
to the age of the animal, or to alterations caused by fossilization, to which I would also 
add changes resulting from attrition, has united these species under the specific name 
B. sepioidea, originally given by De Blainville. Possessing a long series of specimens, 
comprising individuals in different stages of growth, and in which the peculiarities of 
form, taken by M. Deshayes as specific characters, appear to pass gradually into each 
other, I have no doubt as to the propriety of the union, proposed by M. d’Orbigny, of 
the three species above mentioned. That author, however, has added to them a fourth 
species, proposed by M. Deshayes, B. Cuviert ; but which, as it exhibits constant and 
well-defined differences, I think should be retained. 
The rostrum of B. sepioidea is elongated, and pointed at its posterior extremity; 
on the inferior or ventral surface it is sometimes, particularly when young, nearly 
straight, but more frequently it is bent, at about half the length, in an angle more or 
less obtuse, towards the dorsal aspect; the dorsal surface presents a sharp cutting 
edge, slightly arched, and, at its juncture with the callus, exhibits a depression, which 
is strongly marked in mature specimens. At the base, immediately beneath the 
callus, it is more or less dilated, and it is angulated at the margins; the ventral 
surface is more or less convex. The ca//us is narrow, compressed, and deeply rugose ; 
the posterior margin forms an acute angle with the axis of the rostrum, varying con- 
siderably in different specimens. In some instances the inclination of the posterior 
margin is at an angle more or less obtuse with the axis of the rostrum; a condition 
which, as it appears to me, is attributable to the fracture and attrition of the extremity 
of the callus, as the specimens in which this form occurs present a smooth worn ap- 
pearance. The ventral plate, immediately beneath the rostrum, is nearly horizontal, 
but presents a broad undulation, corresponding with the convexity of the ventral 
surface of the rostrum; the lateral extremities, as they approach the sheath, gradually 
diminish in breadth, and assume a nearly vertical position. In consequence of this 
variation in breadth, the ventral plate, which, at the superior margin, is nearly semi- 
circular, presents a regular semielliptical form on the posterior margin. It exhibits 
on the ventral surface a series of sulci, radiating from the apex of the terminal cavity, 
and varying in depth; and it is deeply and sharply denticulated on the posterior 
margin. As the shell enlarges, the plate is thickened considerably by successive 
layers added to the ventral surface. The last layers frequently do not envelope, but 
are a little withdrawn from the margin of, the preceding layers, leaving the previous 
denticulations partly uncovered ; and consequently the margin of the ventral plate, in 
an adult specimen, often presents a double row of denticulations. 
