MACGILLIVRAYIA PELAGICA AND CHELETROPIS HUXLEYI. 
291 
I had not the good fortune to find the vesicular float like that of lanthina, noticed 
by Mr. Macgillivray in the first examples taken ; but in one or two successful hauls 
of the towing-net off the Agulhas bank, Cape of Good Hope, the little animal again 
made its appearance, having been lost sight of on the voyage from Rio to the Cape, 
and the float was found in statu quo, consisting of an aggregation of vesicles varying, 
both in number and size, in different cases. It was exceedingly delicate, and might 
have been easily destroyed or separated from the foot on former occasions by the 
force of the water rushing through the meshes of the net. 
The float of the lanthina has been thought to be an extreme modification of the 
operculum, the absence of which in this genus no doubt has given rise to the idea ; 
but as in the little Macgillivrayia both operculum and float are to be found in the 
same individual, we must admit the latter structure to be quite independent of the 
former, answering a distinctly different purpose. 
The following account of the anatomy of Cheletropis Huxleyi is drawn up nearly 
verbatim from notes made on the examination of the species early in 1853; since 
which time I have met with one or two others of the same genus. 
Numerous specimens of this interesting little mollusk were obtained in Bass’s 
Strait and in the South Pacific, between Sydney and Lord Howe Island. 
Its shell is of a darkish neutral colour, quite transparent, very brittle, and dotted 
all over with minute tubercles. 
The spire is of moderate length, but small compared with the last whorl of the shell, 
which is large and full. 
The aperture is oval, terminating anteriorly in a wide canal or notch. This notch, 
with two others of larger size on the outer lip, and two prominent teeth intervening, 
impart a characteristic appearance to the shell. Leading from the posterior tooth 
on the outer lip, a linear thickening of the shell may be traced quite to its apex. 
The operculum is of an oval form, concentric, developed round a small spiral 
nucleus situate near one extremity, and altogether very much resembles that of 
Atlanta, being also extremely thin, vitreous-looking and brittle. It is not very easily 
detached from the foot for examination, and this circumstance, taken together with 
its extreme minuteness, might explain why it had not been observed by Professor 
Forbes. 
The foot when exposed is proportionably long, rounded at the anterior and pointed 
at the posterior extremity. The whole surface of the disc is closely speckled with 
deep purple pigment-cells, in the centre of which the nuclei remain bright and trans- 
parent, not being obscured by the deposit. The whole surface of the foot is thickly 
covered with extremely delicate and active cilia. 
That portion of the mantle which in Macgillivrayia pelagica forms a long and 
perfect tube, as a respiratory siphon, is short, and the opposite edges are merely 
brought together, without organic union, in the present species. 
The cilia arming this part are much larger than those of the foot just alluded to 
MDCCCLV. 2 R 
