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 SPIRULA and NAUTELUS. 



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 N. P-ompilus of RUMPHIUS 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 in the pre- 

 sent order. 



ORDER 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- 



