MOLLUSCA. 471 



37. ScYLLEA. Branchiae seated dorsally on the fins. 

 Tentacula two. 



On each side of the back are two membranaceous ex- 

 pansions, and one on the tail, supporting on their dorsal 

 surface scattered plumose branchiae. Each of the tentacula 

 is furnished with a large funnel-shaped sheath. The 

 foot is very narrow, with a mesial groove, used in climb- 

 ing up the stalks of sea-weeds. The mouth is placed at 

 the base of the tentacula, and surrounded with a semicir- 

 cular lip. The tongue is in the form of a tubercle, with 

 reflected points. The gullet is plaited longitudinally. The 

 stomach is short and cylindrical, with a ring of hard, longi- 

 tudinal scales. The liver consists of six unequal globules, 

 and the bile is poured into the cardiac extremity of the 

 gullet. The Scyllea pelagica has been long known to na- 

 turalists, and appears to be very common in the equatorial 

 seas, adhering to the stems of the Fuais natans. 



38, Glaucus, Branchiae seated on the margin of the 

 fins. Four simple tentacula. 



On each side of the body there are three or four mem- 

 branaceous expansions, the margins of which are fringed 

 with the simple branchial filaments. This genus was in- 

 stituted by R. FoRSTER, and the oldest known species, G. 

 radiatus, is figured, Phil. Trans, vol. liii. tab. iii. 



(B.) Mouth destitute of corneous jaws, 



39. Thethys. Branchiae forming a rov/ on each side of 

 the back, consisting of fringed processes, alternately larger 

 and smaller. 



The body is ovate, with the cloak and foot continuous. 

 The neck is distinct from the foot, and is narrow. Above, 

 the neck is continuous with the cloak, from which 

 arises a large semicircular expansion, used probably as 

 a fin. The margin of this expansion is fringed with 

 numerous filaments, and on the upper surface, within 



