GASTEROPODA. 55 
Trochus exiguus. I believe the shell which the Crag Polyzoon (C. edaz) has selected for 
its support is a species of Twrritella ; at least in all the numerous specimens I have seen. 
The form it has assumed is unlike that of a turriculated shell, but I think the burden 
imposed upon the animal in its growth, by the Cellepora occupying the base, has 
compelled the Mollusc to expand while the shell was increasing, so as to be deflected 
from its proper angle of volution, and depressed into a turbinated form. Mr. Busk has 
very justly pointed out in the case of Alystdota catena, as in that of other adherent 
Polyzoa, that the animal has the power of eroding the surface of the shell upon which 
it lived, though by what means this is effected is not said. 
I am however inclined to think that the destroyer of the Zurritella was not the Poly- 
zoon, but that the shell has been absorbed or removed by the Mollusc itself in order to 
lighten its heavy and inconvenient incumbrance, for whenever a portion of the shell is 
visible, it has retained its exterior ornament without any apparent abrasion, and in all 
the instances that I have seen, the shell has been a Zurritella. 
I have figured a specimen partially uncovered, obligingly lent to me for that purpose 
by the directors of the Museum at Norwich, which shows a deflection from the normal 
angle of volution (Zurritella incrassata, Supplement, Tab. V, fig. 25 6), and another of 
my own, fig. 25a, wholly enveloped, but distorted. Fig. 3, Pl. XXII, of Mr. Busk’s 
work is, I believe, a Turritella. 
ALVANIA supRANITIDA, S. Wood. Crag Moll., vol. i, p. 99, Tab. XII, fig. 11 @ 6. (as 
A, ascaris). 
ALVANIA SUPRANITIDA, S. Wood. Ann. and Mag. Nat. Hist., vol. ix, p. 534, tab. 5. 
ACLIs -~ Loven. Ind. Moll. Scan., p. 17, 1846. 
— — Forbes and Hanl. Brit. Mol., vol. iii, p. 320, tab. 90, 
figs. 2 & 3. 
—_ _— Jeffreys. Brit. Conch., vol. 4, p. 103. 
Localities. Cor. Crag, Sutton. 
This species was described by me as swpranitida in my catalogue in the ‘ Amn. and 
Mag. of Nat. Hist.,’ of 1842, but in the ‘ Crag Mollusca,’ I (as there stated), in deference 
to the opinion of the late Mr. Alder, referred it to 4. asearis, Turt. The shell swpranitida 
has, however, been recognised by Loven, Forbes and Hanley, and Jeffreys as a living 
species distinct from ascaris, and I have accordingly restored the name. 
Mr. Jeffreys in ‘ Brit. Cone.,’ vol. iv, p. 103, mentions ascaris from the Cor. Crag 
as being in the collection I gave to the British Museum, mixed with swpranitida, but I 
do not recognise it there, nor have I, though it is inserted in the list of Gasteropoda of 
Mr. Prestwich’s Cor. Crag paper, seen it anywhere from the Crag. 
