206 SYNOPTIC COLLECTION. 



probably gave rise to Leptodesma (PI. 393) which has an 

 oblique body with the posterior wing extended, while the 

 anterior border is acute and there is a byssal sinus. 

 These forms may in time have produced another series 

 represented by Aviculopinna and Pinna. 



Aviculopinna (PI. 394) is wedge-shaped and without 

 ears. The two valves are equal and marked by concen- 

 tric lines. The beaks are a little behind the anterior end 

 of the shell. 



Pinna (No. 395, P. rudis Linn.), the probable descend- 

 ant of Aviculopinna, has the same wedge-shaped, equi- 

 valved, and earless shell. The concentric and longitudinal 

 markings are about equally developed. The beaks in 

 Pinna are placed at the anterior end of the shell. The 

 hinge is in a reduced condition, having no teeth, and the 

 substance of the shell is mostly prismatic, very little 

 nacreous matter being found. 



In this form the anterior muscle is four or five times 

 smaller than the posterior, being on the way to a reduced 

 condition. 1 



Other descendants of Nucula, Rhombopteria, and 

 Leptodesma were probably Melina, Avicula, (= Pteria) 

 Malleus, and Ostrea. 



The shell of Melina (No. 396; PL 397, figs, i, 2) is 

 extremely flat, and the posterior part is prolonged to one 

 side making .the shell appear as if deformed. The hinge 

 area (PL 397, fig. 2) has little pits for the ligament which 

 holds the two valves together. 



Avicula^ (PL 398, fig. i, young), has the Nucula-like 

 prodissoconch and the subsequent nepionic stage repre- 

 senting Rhombopteria. It has a straight hinge line, but 

 its posterior wing is extended and there is a deep byssal 

 sinus in the right valve (fig. 2). The triangular pit exists 

 in all Aviculas. The later stage represented by fig. 3 is 

 the Leptodesma stage. The adult has the shell oblique 



1 Sharp, Proc. Acad. Nat. Sci., Phila., 1888, pp. 122, 123. 



