OF MOLLUSCA. 125 



Muzzle of Rissoella opalina produced, rounded, annulate. Ten- 

 tacles four, in pairs close together at the base on each side. Eyes 

 behind the base of the tentacles, only seen through the shell, as in 

 Eulima. Teeth 3 • 1 * 3, in twenty or thirty rows ; central oblong, 

 apex crenated ; lateral inner, broad, denticulated above ; outer nar- 

 rower, curved, denticulated at the tip. 



Dr. Philippi, not knowing that Rissoella and Jeffreysia are syn- 

 onymous, arranges the former in Eulimacea (Moll. Sicil. p. 195), 

 and the latter in Paludinacea (p. 172). 



2. Macgillivrayia. 



Animal ample. Tentacula four, very long, rather broad, linear, 

 rugose (or ciliated ?). Lingual membrane with teeth, like JeffreysiaX 

 Mantle produced into a long siphon. Foot very large, expanded, 

 truncated in front, without lobes, with a float. Operculum semicir- 

 cular, horny, thin, of concentric layers, with faint traces of a spiral 

 structure at the central lateral nucleus, which is on the columellar 

 side ; from it there runs a straight rib or process, continued nearly to 

 the outer margin, and indicated externally by a depression or groove. 

 Shell spiral, dextral, globular, thin, corneous, transparent, slightly 

 concentrically striated, imperforate ; spire not produced, with a sini- 

 stral nucleus ; aperture oblong, entire, angulated below. Peristome 

 incomplete, thin, even-edged. Floating ; gregarious ; furnished 

 with a float like Ianthina 1 



1. M. pelagica, Forbes, I. c. 385. t. 3. f. 8. 



Hab. East coast of Australia, off Cape Byron. 



Mr. Macdonald observes, " The disk of the foot in Macgillivrayia 

 is broad, and connected by a narrow attachment to the body just be- 

 neath the neck ; it carries an operculum behind, and is cleft by a 

 notch in front. A raphe observable in the medial line, as well as 

 indeed the whole character of this part of the organ, seems to shadow 

 forth the transformation of the single foot of the Gasteropod into the 

 wing-like expansion of the Pteropod. Lingual strap with well-marked 

 central and lateral series of teeth, and dentated labial plates. It has 

 a vesicular float like that of Ianthina ; it consists of an aggregate of 

 vesicles, varying both in number and size. It is exceedingly delicate. 

 The gills are fixed to the body immediately behind the head, and 

 not appended to the mantle ; they are four in number and arranged 

 in a cruciform manner round a central point." — Proc. Roy. Soc. vii. 

 191, 1854. 



On further examination the author considers what are here called 

 " naked gills " as probably auxiliary organs of natation, chiefly em- 

 ployed for prehension. — Proc. Roy. Soc. vii. 309, 1854. 



