PHYLOGENY OF THE PELECYPODA. 279 



aim of this paper. It is hoped by tliis means to make connections and differences exist- 

 ing- l)etweL'n some genera better understood, and tluis to further the knowledge of tlie 

 relationships of those genera. So much time has been given to the study of living forms 

 that fossils do not figure as largely as they should in a paper of this natui'e. It is 

 hoped, however, that in future this want may be supplied by investigations in that direc- 

 tion. 



The main line of my research is the study of shell structure and form. Studies of 

 anatomy and habits are subservient and correlative to those of the shell. Facts observed 

 that have no bearing on shell structure are however fi-equently given as they may yet be 

 of interest or of serial value. In studying the shell I have sought especially for young 

 shells hoping in them to find characters of interest which would be valuable in attempt- 

 ing to trace the genetic connection of groups. It is to this study of the young, that 

 Professor Hyatt has especially devoted himself in his researches among the Cephalo- 

 pods, and I owe a debt of gratitude to him for the suggestion to tiy the same method of 

 research in Pelecypods, as well as for the encouragement and sympathy he has given 

 me in my work. 



Theyoung shell of Pelecypods has been very little studied either by itself or in its re- 

 lation to later stages, and I know of no studies embracing observations on the develop- 

 ment of the shell in related genera, or any attempt to trace the connections between the 

 changes of shell form and anatomy during the development of the individual. In Zit- 

 tel's Palaeontology (Barrois' translation) we find in the consideration of the Pelecypoda 

 the following statement: "Quant aux modifications des cocpiilles a leurs ditierentes ages, 

 elles n'ont ete que bien peu etudiees jusqu' iei, et ces observations n'ont pas encore mises 

 a profit dans la systematique." The researches of this paper may therefore be consid- 

 ered as opening up a new field of investigations, as I consider not only the changes in 

 form and structure observed at different periods of development in several genera, but 

 also trace in many cases the serial connection between those changes in form and the 

 phylogenetic history of the group. 



Pelecypods are commonly divided into three groups accordingly as the adults jjossess a 

 single adductor muscle, M m')myai-ians ; two equally de\adoped adductor muscles, Dimya- 

 rians; or two adductor muscles, one large and functionally the most active, the other 

 smaller, Heteromyarians. This classification, I believe, is misleading, as by it are grouped 

 together genera having no clo^e relaLion and similarly ganei'a closely akin are sepa- 

 rated. I show that the size and number of the adductors are closely connected with the 

 relative position of the axes of the body to the hinge axis (section v), and refer to the 

 fiict that the single adductor condition is adopted by several genera not related and 

 which it would be entirely disregai'ding the sense of systematic classification to group to- 

 gether. On this basis of adductor muscle classification even different species of one ge- 

 nus would in one case have to be placed in the Heteromyarians and in anothei- in the Mon- 

 omyarians (see Mytilus, section v, note). The words monomyarian, etc., are occasionally 

 used in the text, but as general descriptive terms, not with the classificatory value ordi- 

 narily applied to them. 



In the text frequent reference is made to the law of concentration and acceleration of 



