434 PHILOSOPHY OF ZOOLOGY. 



corneous ring, furnished with teeth. The sac is furnished 

 with fin-like expansions, and strengthened internally by 



and not muscular, like the sac of the Sepiee, is obvious, from the serpulae 

 which have been found adhering to its surface, and which probably took up 

 their residence after the death of the animal, and the destruction of the soft 

 covering. That the solid part was different in its nature from shell, ap- 

 pears probable, from the circumstance that the latter, when mineralized, is 

 usually converted into calcareous spar, while the foTrner appears of a fibrous 

 structure. 



The alveoli of the belemnite bear so near a resemblance to the species of 

 Orthoceia, that some have concluded that the lattar were originally parts 

 of a belemnite. Several circumstances, however, militate against this opi- 

 nion. Orthoceratites are frequently found fossil, where there are no ves- 

 tiges of belemnites, and even appear to occur in older rocks. Many recent 

 species of the genus Orthocera have been found on our own shores, 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 jirobably, in some respects, inter- 

 nal, — that of the Orthocera was probably external, or covered only by the 

 common integuments. The views here given do not greatly differ from 

 those of Mr Platt, in the Philosophical Transactions, vol. liv. p. 38. 



The Amplexus of Sowerby belongs to this division. It is nearly cylindri- 

 cal, divided into chambers by numerous transverse septa, which embrace 

 each other with their reflected margins. It contains one species from the 

 limestone rocks of Ireland. 



3. Multilocular testacea of a globular form. The first genus of this sec- 

 tion is the MiLioLA. The shell is composed of three or four oval cells, 

 turning round an axis parallel to their longest diameter. Many recent 

 species of this genus are common on our shores : they were included by 

 Montagu in his genus Vermiculum. 



In the Renulina the cells are narrow, linear, unilateral, curved into a 

 part of a circle, and all situate on the same plane. The smallest cell forms 

 a little arch round a marginal axis, and the others are placed contiguous to 

 this on the same side. The species are all fossil. 



The Gyhogona is a shell of a spheroidal form, composed of linear, curved, 

 grooved pieces, terminating in two poles. The external surface is obliquely 

 spiral, the spires terminating at each pole. Found only in a fossil state. 



The 



