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XIII. Remarks on the Anatomy of Macgillivrayia pelagica and Cheletro|jis Iliixleyi 
(Forbes) ; suggesting the establishment of a new Order of Gasteropoda. By 
John D. Macdonald, R.A., Assistant-Surgeon H.M.S. ‘ Herald' Communicated 
by Sir W. Burnett, K.C.B. 
Received November 23, — Read December 21, 1854. 
Having examined the anatomy of the Macgillivrayia pelagica and several 
smaller species of pelagic Gasteropoda, not exhibiting the least similarity in the 
character of their shells, I found that they all presented a very close relationship to 
each other, in the type of their respiratory organs, and in other points of structure of 
less importance. 
The gills in every instance seemed to be fixed to the body of the animal imme- 
diately behind the head, and did not appear to be appended to the mantle, as in the 
Pectinibranchiata properly so called. They were also invariably four in number, and 
arranged in a cruciform manner round a central point. 
They were free in the rest of their extent, elongated and flattened in form, with a 
pointed extremity, and fringed with large flowing cilia, set in a frilled border. They 
were furnished, moreover, with muscular fibres, disposed both transversely and in a 
longitudinal direction, and exhibited great mobility when protruded, but lay side by 
side in the last whorl of the shell when retracted. 
The auditory capsules, each containing a spherical otolithe, were closely applied 
to the inner and posterior part of the larger or anterior ganglion of the subcesophageal 
mass. 
There were but two tentacula, with an eye situate at the outer side of the base of 
each, consisting of a globular lens with distinct optic nerve and retinal expansion. 
The foot was large and very mobile, but a vesicular float has only been observed 
in Macgillivrayia. 
The respiratory siphon was either a simple fold of the mantle, so rolled upon itself 
as to form a temporary tube {Cheletropis), or, as in Macgillivrayia, the borders of the 
fold were united through their whole extent, only leaving a small oblique aperture 
at the end to reveal its true nature. The siphon in Macgillivrayia is about twice 
the length of the shell, and as in the other species examined, beset with vibratile 
cilia. 
A lingual ribbon of good proportional length with well-marked rachis and pleurae 
occurs in all the species, none presenting the character of the broad pavement of 
booklets to be found in the genus lanthina. Very perfect labial plates with a close 
