PIIYLOGENY OF THE PELECYPODA. 



385 



Exogyra is the most highly modified member of the family. Therefore they ai-e con- 

 sidered as dei'ived from Ostrea and are in the ascendent series of this peculiar group. 

 The marked sinuosity iu the left valve of Grypha?a andExogyra is compared to a similar 

 sinuosity in young Ostrea, PI. xxv, figs. 5-6, and is ascribed to the local retraction of 

 the mantle border at that area where excui-reut action takes place (p. 307). 



Exogyra is compared to the adult of species ofChama (the base of the Chamidse) and 

 to the young of Caprinula, PI. xxv, fig. 20, Ilippuiites and Eadiolites (exti-emes of the 

 same group) which in external form it closely resembles. Tne form of all these genera 

 is believed to be built up as the result of cemented fixation with its accompanying in- 

 fluences of environment. When a Pelecypod becomes attached by cemented fixation 

 the first form'induced by the new conditions is a form like Ostrea. A further modifica- 

 tion in this line of variation is a form like Exogyra. The next step iu the same line of 

 variation is a form like Caprotiua or Caprinula, PI. xxvi, figs. 10 and 8. Finally the last 

 step in the same line of variation is a form like Hippurites or Radiolites which are coni- 

 cal or cup-shaped with a considerable degree of superficial radial symmetry (see section 

 VII, and especially foot-note, ]i. 322). 



Alectryonia Avhich is strongly plicated in both valves and adherent by recurved clasp- 

 ing spinous processes, is undoubtedly a side issue 

 from Ostrea and not in the ascendent line of the 

 family. 



I have not had oppoi-tunity to study the PiunidiB 

 satisfoctorily as attempts to ol)taiu very young spec- 

 imens have so far failed. The following is ofiered 

 as the probable genetic relations of the family. Pa- 



1 • /TtT n\ • ^ • „i- f , ,.„.„^„^/I A.^„, Fig. 53. — Tip of yoiins Pinna, sp. indH. from 



IsBopuina (Hall) m lorm is not tar removed trom „ r. ■ ■ ^ ■ v ■> it ■ ■,. -^^ 



i y ' Panama. Origmal in Yale University Museum. 



some Species of Leptodesma and it can be naturally cat. No. 2327. x 4 diam. (Drawn by the au- 

 derived from that genus by reducing the posterior ^^<^'^-) 



extension of the wing and increasing the length of the antero-posterior axis. FromPa- 

 la3opinna or Aviculopinna to Pinna, is a natural step introduced by an increased acute- 

 ness antei'iorly and a greater curvature of the lines of growth posteriorly. In support of 

 this view we find that young Pinnas (20mm. more or less in length), fig. 53, are obtuse 

 anteriorly, and posteriorly have an obtuse abbreviated extension of the hinge line, like 

 Aviculopinna. These features gradually fade out in succeeding growth and give place 

 to the acute anterior and roundly curved posterior border of the valves which is charac- 

 teristic of the genus. The Pinnidaj should, I believe, be considered a degradational 

 group because of the great development of prismatic cellular tissue and loss of nacreous 

 tissue (a weakening character), loss of teeth on the hinge line, and relatively large de- 

 velopment of the anterior adductor muscle (a larval character, highly reduced or want- 

 ing in the ascendent series of related groups). 



Ati'ina is a peculiar genus of the Pinnidse of twisted irregular growth. In the later 

 growth of the shell there is a direct fusion of the two valves both on the dorsal and ven- 

 tral border though in less degree on the latter. In a specimen of A. saccata sp., L. in 

 my collection, 114 mm. in length, the fusion dorsally occupies a length of about 50 mm. 



