290 
MR. J. D. MACDONALD ON THE ANATOMY OF 
file-like arrangement of dental points arm the mouth, in some instances at least, but 
most probably in all. 
It is very remarkable, that the little animals possessing all the characters just 
detailed in common, should fabricate shells so very different in general form and 
other particulars as to permit of their arrangement into well-marked genera. 
The obvious difference existing between the pectinibranchiate type of respiratory 
organs and that of the little Gasteropoda now under consideration, must at once 
afford sufficient grounds for placing the latter in a distinct order by themselves ; in 
illustration of which, I have selected the anatomy of MacgilUvrayia pelagica and 
Cheletropis Huxleyi, whose shells have been already described by Professor E. Forbes, 
and figured in Mr. Macgillivray’s ‘Narrative of the Voyage of H.M.S. Rattlesnake.’ 
The beautiful little Gasteropod named after the latter gentleman, who was its first 
discoverer, was originally believed to be purely Australian, but upwards of a dozen 
specimens were captured in the towing-net, about the latitude of Bahia, while 
H.M.S.V. ‘Torch’ was proceeding to Rio de Janeiro. 
The disc of the foot when expanded was of considerable breadth, but its attach- 
ment to the body was small and situate just beneath the neck {Trachillpoda). Its 
lateral borders were united posteriorly, forming a rounded extremity, to the upper 
surface or heel of which the concentric horny operculum with spiral nucleus was 
attached, but they were notched in front, so that the angles between them and the 
anterior margin, which was slightly convex, were prominent and pointed a little 
backwards. The raphe to be noticed in the mesial line, and in fact the whole 
character of this part of the organ, seemed to shadow forth the transformation of the 
single foot of the Gasteropod into the wing-like expansions of the Pteropod. The 
mouth of the animal was furnished with two horizontally placed, crescentic plates, 
adapted for acting upon one another in breaking up food. 
The lingual strap bore many points of analogy to that of the Heteropods, the 
single series of plates in the rachis being angular with a finely serrated border, and 
the pleurse consisting each of three rows of simple uncini, or with delicate teeth on 
the concave border. 
The eyes, which were distinctly to be seen with a common lens, were surrounded by 
a rose-coloured zone, giving them a remarkable appearance. The four naked branchiae | 
fringed with gracefully curved cilia of unusually large size, radiated from a point at i 
the back of the head like so many feathers set in a crown, which when taken together ! 
with the glowing eyes and brown labial teeth of the little mollusk, imparted quite a | 
singular aspect to the whole physiognomy. 
On the left side of the body a tubular process of the mantle protruded from the 
shell, and seemed to indicate the coexistence of a respiratory chamber with naked 
branchiae. The length of this siphon nearly equalled that of the foot, and its aper- 
ture was oblique, as that of a portion of the mantle rolled into a tube would natu- 
rally be. 
