146 MR. T. D. A. COCKERELL ON CLAUSILIA RUGOSA. [Feb. 17, 



shell, which remains uninjured. Instead of merely repairing the 

 wall of the shell, the animal has constructed a new aperture, which, 

 although not so perfectly formed as the first one, still has the 

 lamellae and general structure of the normal mouth. xV similar 

 monstrosity is figured by Moquin-Tandon, Hist. Nat. Moll. France, 

 pi. xxiv. fig. 19. 



The other two specimens (fig. 3) are noticeable because, although 

 from the same place, they differ so much in size, form of the aperture. 



b " 



Fig. 2. 



X2 



h a 



Fig. 3. 



A B 



Specimens of Clausilia nigosa from Isleworth, Middlesex. 



Figs. 1, 2. Specimen with two apertures, the new one having been formed after 

 a fracture of part of the body-whorl. 



Fig. 3. Specimens having different characters, A being fully developed, and B 

 probably stunted by an algoid growth. — A. Nearly symmetrically 

 pyriform aperture of clean specimen, 12 millim. long; B. Non- 

 syiiimetrically pyriform aperture of specimen coated with alga, 

 and 10 millim. long. 



and general structure that they look almost like examples of different 

 species. It will be noticed that the larger specimen, 12 millim. Ion?, 

 with the nearly symmetrically pyriform aperture, is clean ; while the 

 smaller one, 10 millim. long, with the asymmetrically pyriform aper- 

 ture, is coated with an algoid growth, which was green and conspi- 

 cuous when the shell was found. Possibly this affords us a clue to 

 the reason of the differences between the shells. Messrs. Bornet and 

 Flahault (Bull. Soc. Bot. France, 1890) have been investigating 

 certain parasitic algse which live in shells, and, penetrating under the 

 e[)idcrmis, destroy the hard structures by degrees. Tiiis has been 

 observed in marine and freshwater shelLs, and is very possibly one 



