Ill 



obtained, before we can form any opinion respecting the circumstances 

 in the economy of the animals inhabiting them, which have demanded 

 these particular modifications. 



The fossil shells of this genus are, 1 believe, always found imbedded 

 in a matrix, excepting those fragments of the straight part, which are 

 perhaps not to be distinguished from orthoceratites, the genus which 

 must next employ our attention. Spirulites are found in chiefly a red- 

 dish marble, in Mecklenbourg, in some parts of Normandy, and, it is 

 also said, in Switzerland. 



But they are obtained, by far most frequently, from Gothland and 

 Oelarid : the latter place affording the finest specimens : those in which 

 the spiral part of the shell is preserved are very rare. The one which 

 is figured Plate VI. Fig. 11, is from Oeland, and was purchased from 

 the Calonnian collection. In this specimen, not only are the spiral turns 

 of the shell seen, but traces of the continuous shelly siphunculus also 

 are evident, Plate VII. Fig. 18, in a dark red lime-stone, evidently 

 containing a large proportion of iron ; in which is displayed the spiral 

 termination,, and a small part of the straight portion of the last turn 

 of one of these shells. Plate VII. Fig. 19, represents another of these 

 fossils, imbedded in grey marble, from some part of Germany. 



LXXFV. Orthocera. A straight or slightly bent, rather conical, mul- 

 tilocular shell ; the chambers separated by transverse curved septa, 

 pierced by a tube. 



The shells of this, as well as those of the former genus, were 

 placed by Limiaeus under the genus Nautilus; the considerable dif- 

 ference, which is observable in their external form seems, however, 

 fully to warrant their separation. Most of the shells, which, though 

 straight, have been considered by Linnaeus, and other naturalists, as 

 nautili, are minute and even microscopic shells ; and, when sufficiently 

 examined, some of them may be found to possess characters distinctive 

 even from both those of Nautilus and Orthocera. These are N. jugosus, 

 N. costatus, N. radicula, N. subarcuatulus, N. semilituus, N. rectus, N. spi- 



