Many species of multilocular testacea occur in a fos sil slate, which are usually considered as the remains of cephalopodous mollusca. Some of these are spiral like the Nautilus, others are orbicular like the Miliola, while in a third the chambered shell is nearly straight, as in the Orthocera.
As intimately related to the last mentioned genus, we may here take notice of the belemnite. In this fossil the apex is solid, with a groove or fold on one side, and at the thick end there is a conical cavity filled with a shell divided into chambers, which are penetrated by a pipe. If we regard this body as the remains of a ce phalopodous animal, we may consider the exterior solid extremity to have been a corneous covering, and the chambered alveolus as the seat of the body of the animal, which likewise enveloped the base. That the solid base was hard, and not muscular, like the sac of the Sepiacea, is obvious from the surpulR which have been found ad hering to its surface, and which probably took up their residence after the death of the animal, and the destruc tion of the soft covering. That the solid part was dif ferent in its nature from shell, appears probable from the circumstance that the latter, when mineralized, is usually converted into calcareous spar, while tke former appears of a fibrous structure.
The alveoli of the belemnite bear so near a resem blance to the species of Orthocera, that some have con chided that the latter were originally parts of a belern nite. Several circumstances, however, militate against this opinion. Orthoceratites are frequently found fossil where there are no vestiges of belemnitcs, and even ap pear to occur in older rocks. Many recent species of the genus Orthocera have been found on our own shin es without the vestige of an external covering. Had they possessed any such solid apex, like the belemnite. it is probable that it would have been detected in the recent kinds, since it is sufficiently durable to retain its form in the solid strata. The shell of the belemnite was pro bably, in some respects, internal—that of the orthocera was probably external, or covered only by the common integuments. The views here given do not greatly dif fer from those of Mr. Plat, in the Philosophical Transac tions, vol. liv. p. 38.