132 MOLLUSCA. 
the animals of two of the genera, to furnish some hints for 
those who are fond of classifying animals from their analo- 
gies. These genera are SrrruLA and NavrTitus. 
In the Spirula, the shell, which is concealed under the 
skin of the back, is spiral, with the whorls separate, the 
mouth orbicular, the chambers perforated by a pipe, and 
the last cell produced into a tube. The position and use 
of this terminal tube are unknown. The S. vulgaris is the 
most common species, and inhabits the seas in the West 
Indies. In the restricted genus Nautilus, the shell is sup- 
posed to be external, and the body of the animal to be lodg- 
ed in the last chamber, and to be fixed by a ligament which 
descends into the central pipe. In the shell itself, the turns 
of the spire are contiguous, and the last whorl embraces the 
others on the sides. The WN: Pompilus of Rumputvs is the 
only species in which the animal has been detected. 
The other genera which have been formed in this order 
depend exclusively on the characters furnished by the shells ; 
and the resemblance which these bear to the preceding 
genera, constitutes all their claim to be included im the pre- 
sent order. 
OrvDER II.—SEPIAcEA. 
Destitute of a multilocular shell. 
The sac is strengthened by horny or testaceous plates, 
unless where the habits of the animal render such support 
unnecessary. 
1. Head surrounded with eight arms and two feet. 
The two feet are nearly similar in their structure to the 
arms, or tentacula, but considerably larger in their dimen- 
