PHYLOGENY OF THE PELECYPODA. 381 



is a discrepancy in that the right valve of these genera has a well defined byssal sinus, 

 whereas no byssal sinus exists in any of the figures of Rhombopteria cited. Dr. Beecher 

 writes me that young Avicula? demissa has a byssal sinus and this species is a near ap- 

 proach to Rhombopteria, es|)ecially when young, bnt differs in having the posterior wing 

 produced in the adult. It is quite possible that species of Rhombopteria may be found 

 which have a byssal sinus; bnt if no such species should be found, it would not seriously 

 affect our comparison of young Aviculas, etc., to this early type. The byssal sinus acquired 

 in later periods of the development of a gi-oup might well be pushed back to the nepionic 

 stages in the ontogeny of an individual by acceleration of development, also the exist- 

 ence of the character is quite variable in some genera, as in Aviculopecten. 



The genetic i-elations expressed in the following pages and table must be looked at as 

 a first attempt to classify the various groups and genei-a in their serial connections, for 

 such I believe it is. Changes may and probalily will be necessary in parts, but as a 

 whole I believe it represents a close approximation to the true phylogenetic sequence of 

 the genera considered. 



The dimyarian prodissoconch characteristic of the early stages of this whole group is 

 the representative in development of a dimyarian Nuculoid radical as discussed in sec- 

 tion XV. Therefore a Nuculoid form makes a basis from which to start our jjhylogenetic 

 series. 



These studies tend to show that the primitive Aviculoid ancestor came off directly from 

 the ]S"uculoid radical because the first nepionic stages of Avicnla, Leptodesma, Perna, 

 Pecten, Aviculopecten, etc., correspond closely with the earliest and simplest known 

 Aviculoid. If intermediate forms existed between the IS^uculoid radical and the earliest 

 known Aviculoid, traces of it are not indicated in the shell, so that such possible transi- 

 tional steps cannot be ascertained with our present knowledge. Therefore Rhombop- 

 teria, as the most primitive known Aviculoid, is considered the next step upward in the 

 jihylogeny of the group. 



Fi-om this point up the phylogeu}" of the various groups is on divergent lines as ex- 

 pressed in the tabular view, and in the following pages we shall consider the groups iu 

 succession as indicated in the table, p. 391. 



Leptodesma,^ Hall, is a genus embracing a group of Devonian Aviculoids characterized 

 by an oblique body. The posterior wing is more or less produced beyond the borders 

 of the valves; anteriorly the shell is acute or nasute. The hinge is narrow with a slen- 

 der, lateral tooth. The posterior adductor muscle is lai-ge, subcentral, but the anterior 

 is not figured by Hall and is therefore presumably very reduced. The genus forms a 

 natural passage fi-om Rhombopteria to the true Aviculas. As Professor Hall saj^s, the 

 species present a remarkable assemblage of forms. At one extreme the shell has a well- 

 developed posterior wing, acute anteiior border and byssal sinus closely resembling Avi- 

 cula. From this extreme it passes "" . . . through various phases ui form, proportions, 

 etc., until the wing becomes nearly obsolete, the byssal sinus obscure and the anterior 

 end rounded. ..." This other extreme brings the form very close to our genus 



' For a consideration of this and other palaeozoic genera Yorlj, Vol. v, Parts i-n. I am also recently indebted to 



I am greatly iudeOited to Professor HaU's fine work on that venerable pateontologist for tlie opportunity to study 



IV^vouiau Lauiellibniiu-hs in the Natural History of New liis collections and types at Albany. 



MKMOIUS lIOSrON SOC. NAT. lllSl'.. VOL. IV. 51 



