416 Dr. J. E. Gray on tJie Tongues of Mollusca. 



have a collection of them formed with great care, with all their 

 habitations most accurately marked and arranged strictly geogra- 

 phically, or rather in natural geographic stations ; and when this 

 has been done, we shall be surprised to find how we have been 

 manufacturing species which nature never intended ; and on the 

 other hand, equally surprised how we have associated specimens 

 together as one species which are most distinct from each other. 

 As an instance of the latter kind, I may cite Crepidula ungui- 

 formis of authors. This shell is said to be found in the Medi- 

 terranean, on the north-east coast of America, the West Indies, 

 the southern and the tropical portions of the west coast of Ame- 

 rica. In all these localities it is found on the inner surface of 

 shells ; being attached to a concave surface, it is flat or slightly 

 concave externally, and is always of a white colour, like most 

 shells which live in a situation where they are not exposed to 

 the light. Believing that this form and colour are caused by 

 the situation in which it is found, I feel convinced that the 

 Crepidula unguiformis of Sicily is an accidental variety of the 

 usual-shaped Crepidula of the Mediterranean seas ; and that the 

 same is the case with the specimens which have been called C. 

 unguiformis from other seas ; and if the natural-formed species 

 of these countries are distinct, which I believe is now universally 

 admitted, the flat, un coloured varieties of them are equally dis- 

 tinct ; though I am quite willing to own that I know no character 

 or mark on the shell by which the monstrosities from the diff'erent 

 localities can be distinguished from each other when placed side 

 by side in the cabinet. 



Many conchologists, especially those who collect the specimens 

 from their native habitat, assert that certain specimens are a 

 most distinct species, because they are always found in a peculiar 

 locality, when it is the locality to which they are attached which 

 gives them the peculiarity of form or colour : thus, the C. ungui- 

 formis must be a species because it is found on the inside of the 

 shell, is flat and is white, whereas the colour and form depend on 

 the locality. Specimens are, however, rarely found which were 

 flat or concave externally, and white when young, and are convex 

 and brown-rayed when adult, or vice versa ; the animal having 

 changed its locality during its life. In the same way, others assert 

 that Crepidula incurva is very distinct, because it is very convex 

 with a narrow base, and is always found attached on shells, and 

 is generally crowded together one on another ; the narrow shape 

 being produced by the shape of the shell, and the convexity of 

 the back by the convexity of the back of the shell entering into 

 and pushing up the cavity of the specimen which is attached 

 to it. 



These are treated by many conchologists as theories, but they 



