436 PROCEEDINGS OF THE ACADEMY OF [1898. 



the animal and shell become long and narrow ; if growth is limited 

 in front, the shell becomes short and broad ; if limited on all sides, 

 the shell may increase greatly in thickness but remains small in area, 

 completely filling the space in which it is found. In such cases, the 

 lines of growth are crowded close together, and the very edge of the 

 shell may be as thick as any other portion. 



" The cause of these variations is not far to seek ; the shape of the 

 shell is conditioned by the shape and position of the mantle edge; 

 the mantle is moulded over the surface upon which the animal rests; 

 and consequently the shape of the shell comes to correspond in time 

 to any sort of a surface upon which the animal is attached." 1 



Arnold Graf 2 has described a case in which a shell of C.forni- 

 cata was marked by radial folds corresponding to those of a Pecten 

 upon which the Crepidula was attached. I have, myself, repeatedly 

 noticed similar cases. 



More recently, Bradney B. Griffin, 3 has called attention to a Pla- 

 cuanomia shell which was found attached to a valve of Saxidomus, 

 and which was marked by lines and folds exactly coinciding with 

 the concentric markings of the Saxidomus. 



Griffin also remarks that many similar phenomena have been ob- 

 served and commented upon by paleontologists in fossil shells, and 

 he refers particularly to two papers by Keyes 4 on the modifications 

 of Platyceras shells due to the surface of attachment. 



All these modifications are similar to those which I have observed 

 in Crepidula, and are, undoubtedly, due to the causes which have 

 been mentioned. 



Such irregularities of form could scarcely be called dimorphism, 

 though they might properly enough be called environmental poly- 

 morphism. In no case which I have observed is there any evidence 

 that any of these modifications of form are becoming hereditarily 

 fixed, though they may be found in many individuals and have fre- 

 quently been considered of specific value (e. g., Crepidula glauca 

 and C. acida). 



1 Conklin, The Embryology of Crepidula. Jour. Morph., Vol. XIII, 1897. 



2 A. Graf, Adaptation of the shells of Crepidula jomicata to the shell of Pec- 

 ten Jacobeus. Trans. New York Academy of Sciences, April 3, 1896. 



'Griffin, Adaptation of the shell of Placuanomia to that of Saxidomus with 

 remarks on shell adaptation in general. Trans. New York Academy of Sci- 

 ences, Feb li2, 1897. 



4 Keyes, The Sedentary Habits of Platyceras, Am. Jour, of Science, October, 

 1888, and On the Attachment of Platyceras to Palaeocrinoids and its effects in 

 modifying the forms of the Shell, Proc Am. Phil. Soc , Vol. XXV, 1888. 



