PHYLOGENY OF THE PELECYPODA. 381 



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

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

 writes me that young AviculaP demissa has a byssal sinus and this s^jc'cies is a near ap- 

 proach to Rhombopteria, esi'ceially when young, but ditfers in having the posterior wing 

 jiroduced in the adult. It is quite possible that species of Khombopteria may be fouiul 

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

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

 in later periods of the development of a group might well be pushed back to the ne[)ionic 

 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 relations expressed in the following pages and table must be looked at as 

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

 such I believe it is. Changes may and probably 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 jii'odissoconch characteristic of the early stages of this whole group is 

 the representative in development of a dimyarian Nucnloid i-adical as discussed in sec- 

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

 series. 



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

 the ]!iuculoid radical because the first nepionic stages of Avicula, Leptodesma, Perna, 

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

 Aviculoid. If intermediate forms existed between the IN^uculoid I'adical 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 oiir present knowledge. Therefore Rhombop- 

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

 phylogeny of the group. 



From this point up the phylogeny of the various groups is on divergent lines as ex- 

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

 succession as indicated in the table, p. 391. 



Leptodesma,^ Hall, is a genus emliracing 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 nasutc. The hinge is narrow with a slen- 

 der, lateral tooth. The posterior adductor muscle is large, subcentral, but the anterior 

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

 natural passage from 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 anterior border and byssal sinus closely resembling Avi- 

 cula. From this extreme it passes '' . . . through various phases in form, propoi-tions, 

 etc., until the wing becomes neai-iy 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 ami otUoi-paUvozoic genera York, Vol. v, I'arts l-n. I am also recently indebted to 



I am greatly indebted to Professor Hall's line work on that venerable palaeontologist for the opportunity to study 



Devonian !^aniellil)ranchs in the Natural History of New his collections and types at All)any. 



MKMOLUS BOSTON SOC. NAT. UISl'., VOL. IV. 01 



