NOTE ON C/ECUM TRACHEA (MONTAGUE), CsECUM POLLICARE, 

 CARPENTER, AND CECUM VITREUM, CARPENTER. 1 



From a number of Csecums recently received from Algiers, I have been able to select 

 a series of Ccecum traclicea (Mont.), upon which it was easy to make the following observa- 

 tions : — 



(1.) The specimens from the coast of Algiers appear to be much smaller than those 

 commonly taken on the shores of Great Britain and of the Bay of Biscay. 



(2.) The characters become gradually less marked on the members of the series, and 

 eventually specimens are found which present no other features than those observed in 

 Ccecum pollicare, Carp., which they further resemble in size. The ornamentation finally 

 vanishes, and the shell, which has thus become smooth, shows no more sculpture than 

 does a specimen of Ccecum vitreum of the same author. 



A large number of specimens of Cascum, which were collected on board the " Travail- 

 leur " and the " Talisman " in the Strait of Bocayana, between Lanzerote and Fuerta- 

 ventura, were recognised as belonging to the two species, Ccecum pollicare and Ccecum 

 vitreum. 



It was easy to observe on a series of specimens that the former gradually became 

 attenuated and lost its principal character, the longitudinal ornamentation, and arrived at 

 the smooth condition seen in CcBcum vitreum. 



A similar phenomenon was noticed in the case of Ccecum achironum, from the coast of 

 Brazil, which, while very different in form, is ornamented in the same way as Ccecum 

 pollicare. It also passes gradually into the smooth state by a weakening of the longi- 

 tudinal markings ordinarily so prominent in typical specimens. 



Thus the characters of Ccecum pollicare seem to connect it closely with Ccecum tracliasa; 

 it has the same form, the same aperture, the same septum, and sometimes the appearance 

 of rings upon the earlier portions of the tube. But although the large series of specimens 

 from Algiers apparently indicates that the one species is derived from the other, yet 

 Ccecum traclicea has never been found on the coasts inhabited by Ccecum pollicare. 

 Furthermore, the degeneration of the surface ornamentation does not cease until Ccecum 

 pollicare passes into Ccecum vitreum, which also exhibits the same form, the same 

 aperture, and the same septum, and differs only in the fact that its surface has become 

 smooth. 



1 Added by the Marquis de Folin, May 8, 1886. 



(ZOOL. CHALL. EXP. — PART XLII. — 1886.) Tt 87 



