MOLLUSCA. 463 



to several chambers, which communicate with each other by 

 triradiated apertures. It is uncertain whether the animal is 

 to be considered as dextral or sinistral. This genus was 

 instituted by us several years ago, for the reception of the 

 Nautilus lacustrisoi Lightfoot, first described and figur- 

 ed in Phil Trans, vol. Ixxvi. p. 160. Tab. 1. f 1. 8. 



2d Tribe. 



Shell simply conical. 



29. Akcylus — Foot short, elliptical. 



The tentacula are short, compressed, and a little trun- 

 cated. This genus was formed by Geoffrov, and includes 

 the Patella lacuslris and ohlonga of British conchologists. 



2d Subdivision. 



Body destitute of the external protection of a shell. 



30. Peronia. — Head with two long retractile tentacula. 

 The snout is divided into two broad appendages. Between 

 the tentacula, towards the right side, is the opening for the 

 penis. The anus is terminal, immediately above which is 

 the entrance to the pulmonary cavity ; and on the right is 

 the opening to the female organs, from which a groove runs 

 towards the right lobe of the snout. 



The mouth is destitute of a proboscis or jaws. The tongue 

 is merely a cartilaginous plate grooved transversely. The 

 gullet is long in proportion, with a villous surface. There 

 are three stomachs, each distinguished by its peculiar cha- 

 racters. The first is a true gizzard, covered internally with 

 a cartilaginous cuticle, and its walls formed of two strong 

 muscles, with connecting ligaments. The second stomach 

 is funnel-shaped, with prominent ridges both on its external 

 and internal surface. These ridges, at their origin inter- 

 nally, are highest, and project considerably into the cavity, 

 acting like a valve in retarding the progress of the food. 

 The third stomach is short and cylindrical, covered inter- 

 nally with equal longitudinal fine ridges. The intestine is 



