280 ROBERT TRACY JACKSON ON THE 



developmout. This law was enunciated separately and almost simultaneously by Pro- 

 fessors Hyatt (29)' and Cope. The law is, that cJutracteristics acquired in the adult, if 

 inherited at all, are inherited at earlier and earlier periods in successive generations, until 

 they exist only in enibryonic stages, or are completely lost through disuse and concentra- 

 tion and acceleration of development. This law finds strong support in the facts pre- 

 sented in the development of the oyster and other Pelecypods as well, where many pe- 

 culiarities occur which are well explained by it. A corollary of the law of concentration 

 and acceleration of development is that, in the young stages are found the equivalents of 

 which are to he sought in the adults of related groujjs. 



I have met with difficulty in pi'ocuring young specimens of Pelecypods to study, for the 

 most part depending on what I collected in the field or found adhering to masses of shells 

 in the museums. Conchologists commonly pay little attention to the young in their col- 

 lections as they are not considered characteristic. They certainly are not characteristic, 

 in that they have not a full expression of features found in the adult, but they are char- 

 acteristic of their period in the development of the individual and from this point of 

 view they are well worth collecting and studying carefully. 



In section in Professor Hyatt's classification of stages of growth and decline is con- 

 sidered in its application to mollusca. I have there qualified some of his definitions 

 and introduced a new term, jihylemhryo, which is applied to the stage in development 

 where features exist which first render the embryo referable to the class or phylum to 

 which it properly belongs. In considering stages of growth it is a valuable assistance 

 to have an established nomenclature by which any given period may be briefly and defi- 

 nitely expressed, and that given in this paper seems to fill the requirements of such a 

 nomenclature. 



The phylembryonic stage of developing Pelecypoda presents some striking features 

 which have not been previously considered as far as known. Tiie shell of this period is 

 characterized by having a straight hinge line as shown in many genera(fig. 25, p. 300, PI. 

 xxiii, figs. 7-11 and PI. xxiv, figs. 13-15), although later succeeded by a curved one. 

 An important point is the fact that at this age but one adductor muscle exists, and that 

 one is destined to become the anterior adductor of adults which retain it(dimyarians). 

 The posterior adductor is developed later. The mouth and anus in early embryonic 

 stages of Pelecypods are closely related on the ventral border of the embryo (fig. 23, 

 p. 299) and the anterior adductor muscle develops early on the dorsal side of the ali- 

 mentary canal (fig. 25, p. 300; PI. xxiii, figs. 9-11) which position it retains through- 

 out life, in dimyarians. The posterior adductor is not developed until the anus has re- 

 volved dorsally from its early ventral position, and then the adductor is formed on the 

 ventral side of the alimentary canal, which position it retains throughout life in all 

 Pelecypods. We see, therefore, in this an explanation of the relation of the adductor 

 muscles to the alimentary canal, one adductor overlying, the other underlying it, PI. 

 XXV, figs. 9-12 (for full discussion see section v). 



•A list of publications quoted or otlierwise rcfei-red to is as in tliis case correspomls with a iiumbei- on the refer- 



given ill section xvir. Wliere the reference in tlie text is euce list. 



not peifecUy evident, a number in brackets is used which 



