GASTEROPODA. 55 



Trochus exiguus. I believe the shell which the Crag Polyzoon (C. edax) has selected for 

 its support is a species of Turritetta ; 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 Alysidota 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 Turritetta 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 Turritetta. 



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 frpm the normal 

 angle of volution {Turritetta incrassata, Supplement, Tab. V, fig. 25 b), and another of 

 my own, fig. 25 a, wholly enveloped, but distorted. Pig. 3, PI. XXII, of Mr. Busk's 

 work is, I believe, a Turritetta. 



Alvania supranitida, S. Wood. Crag Moll., vol. i, p. 99, Tab. XII, fig. 11 a b. (as 



A. ascaris). 



Alvania supkanitida, «S. Wood. Ann. and Mag. Nat. Hist., vol. ix, p. 534, tab. 5. 

 Aclis — Loven. Ind. Moll. Scan., p. 17, 1846. 



— — Forbes and Hani. 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 supranitida in my catalogue in the ' Ann. 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 A. ascaris, Turt. The shell supranitida 

 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 supranitida, 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. 



