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. longirosiris, 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. Cuvieri ; 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 callus 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. 



