280 ROBERT TRACY JACKSON ON THP: 



develop iTi3iit. This law was enunciated separately and almost simultaneously by Pro- 

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

 inherited at all, are inherited at earlier ani earlier p3riods in successive generations, until 

 they exist only in embryonic 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 be sought in the adults of related groups. 



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

 most part depending on Avliat 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 charactei-istic, 

 in that they have not a fuUexpression of feattires found in tlie 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 moUusca. I have there qualified some of his definitions 

 and introduced a new term, phylembryo, 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 i)hylembryonic stage of developing Pelecypoda presents some striking features 

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

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

 xxin, 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(dimyariaus). 

 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 eai-ly 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 alimentai-y 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 otherwise I'efei'i'L'd lo is as in tliis case correspoiid-i Willi a imniber on the reTur- 



given in section xvii. Wlicre tlie reference in the text is ence list, 



uot perfecUy evident, a uninlier in Ijrackets is used wliich 



