636 



MEMOIRS OF THE CARNEGIE MUSEUM. 



1913). One shell, female, with soft parts, and soft parts of a male and female with- 

 out shells. 



Distribution. — From the upper Amazon drainage in Peru and Ecuador through 

 Colombia into Central America, northward to Guatemala, where it is found both 

 on the Atlantic and Pacific slojx's. 



Von Ihering's key has failed me in the identification of this species, since he 

 uses size as a criterion, while my specimens are all rather small. The height, 

 also used as a distinctive character, is apparently unreliable, as shown by my 

 specimens. Therefore this group [Ith in the key) needs revision. 



Nevertheless the two largest specimens at hand (one from the upper Amazon, 

 the other from Guatemala) agree fairly well with Von Ihering's account and 

 measurements, and also with those of Von Martens. There is no question about 

 the identity of the Central American specimens with the form from northern South 

 America. The posterior adductor-scar is said to be subsinual (Von Ihering p. 118). 

 This fits my smaller specimens from the upper Amazon, while the larger one is 

 peculiar. It appears as if there were in each valve two superimposed scars, the 

 one more anterior, the other a little farther back. The latter is superficial, but ap- 

 parently corresponds to the latest growth-addition to the shell. Its anterior end 

 is slightly in advance of the ligamental sinus (presinual). In this specimen, how- 

 ever, there is a disturbance of the regular growth, as indicated by a strong growth- 

 rest on the outside of the shell, and the shell looks as if stunted behind. This 

 individual is therefore not normal. In the large specimen from Guatemala, the 

 scar is also slightly presinual (about one-third of the retractor-process projecting 

 in front of the sinus), and in Von Martens' figure (representing a very large speci- 

 men) this scar is still more presinual. 



The sinuosity of the lower margin is seen in mj' two larger specimens, but is 

 not so strong as in Sowerby's and Von Martens' figures. The young specimens, 

 which undoubtedly belong with the larger one, show hardly a trace of this sinuosity. 



