PIIYLOGENY OF THE PELECYPODA. 



385 



Exogyra is the most highly modified meml)er of the family. Therefore they are con- 

 sidered as derived from Ostrca and are in the ascendent series of this peculiar group. 

 The marked sinuosity in the left valve of GryphaeaandExogyra 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 excurrent action takes place (p. 307). 



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

 to the young of Caprinula, PI. xxv, fig. 20,Hippurites and Eadiolites (extiemes 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 in the same line of 

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

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

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

 VII, and especially foot-note, p. 322). 



Alecti-yonia which 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 opportunity to study the Pinnidse 

 satisfactorily as attempts to obtain ver}' young spec- 

 imens have so far failed. The following is oftered 

 as the probable genetic relations of the family. Pa- 



1 • /TT ii\ • c • I. 4" „ „ „,1 ■(■..„.„ Fig. 53. — -Tip of vouiia; Pinna, sn. indH. from 



Iseopnina (Hall) m form is not lar removed irom „ ,i • • , • ^- ■, n ■ ■, .r 



i V / Panama. Ongiiial in i ale University Museum. 



some sj^ecies 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- 

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

 ness anteriorly and a greater curvature of the lines of growth posteriorly. In support of 

 this view we find that young Pinnas (20 mm. 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 gi-adually fade out in succeeding groAvth and give place 

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

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

 group because of the great development of pri.smatic 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). 



Atrina is a peculiar genus of the Pinnidae of twisted irregular growth. In the later 

 growth of the shell there is a tlirect 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. 



