tCINETOGENESlS. 267 



pelecypods which are attached to a foreign object of 

 support by the cementation of one valve. All are 

 highly modified, and are strikingly different from the 

 normal form seen in locomotive types of the group. 

 The oyster may be taken as the type of the form 

 adopted by attached pelecypods. The two valves are 

 unequal, the attached valve being concave, the free 

 valve flat ; but they are not only unequal, they are 

 often very dissimilar, as different as if they belonged 

 to a distinct species in what would be considered typ- 

 ical forms. This is remarkable as a case of acquired 

 and inherited characteristics finding very different ex- 

 pression in the two valves of a group belonging to a 

 class typically equivalvular. The attached valve is 

 the most highly modified, and the free valve is least 

 modified, retaining more fully ancestral characters. 

 Therefore, it is to the free young before fixation takes 

 place and to the free, least-modified valve that we 

 must turn in tracing genetic relations of attached 

 groups. Another characteristic of attached pelecy- 

 pods is camerated structure, which is most frequent 

 and extensive in the thick attached valve. The form 

 as above described is characteristic of the Ostreidae, 

 Hinnites, Spondylus, and Plicatula, Dimya, Pernos- 

 trea, Aetheria, and Mulleria; and Chama and its near 

 allies. These various genera, though ostreiform in 

 the adult, are equivalvular and of totally distinct form 

 in the free young. The several types cited are from 

 widely separated families of pelecypods, yet all, under 

 the same given conditions, adopt a closely similar 

 form, which is strong proof that common forces acting 

 on all alike have induced the resulting form. What 

 the forces are that have induced this form it is not 

 easy to see from the study of this form alone ; but the 



