THE NAUTILUS. 117 



remote from the so-called ^'■Hyriince" of Asia, at least from the two 

 investigated genera, Parreysia and Lamellidens. 



If Spatha is to be placed in a family different from the Unionidce, 

 Hyria has to go out of the Unionidct also, and has to remain with 

 Spatha. Yet the differences from Spatha are such that possibly the 

 separation into two subfamilies would be advisable. This suggestion 

 will be sii[)ported by the following observations: 



Tetraplodon undosus (v. Mart.).' 



This species was collected by Mr. J. D. Hasenian in the Rio Ti6t6, 

 25 miles above Itapura, State of Sao Paulo, Brazil, on "silty river 

 banks," Sept. 27, '08. There are several lots at lian<l, and the one 

 more closely investigated consists of four specimens with the soft 

 parts, among them males and sterile females. 



The structure of the soft parts of this species is essentially identical 

 with that of Hyria, but 1 note the following differences (see pi. VI, 

 fig. 7, and pi, VII, fig. 7): 



Branciiial opening closed in front by the firm union of the inner 

 mantle edges (x in fig. 7, pi. ii). This connection is rather short, 

 and restricted to the edge, and does not continue inward. It is easily 

 torn by rough handling. Further in front the mantle edges are 

 smooth and unconnected. 



The palpi are rather wide and slightly produced posteriorly, with 

 a blunt point, thus becoming almost semilunar, but not falcate; their 

 posterior edge is connected for about one-third. 



The anterior end of the inner gill is in contact with the posterior 

 end of the inner palpus, but not connected with it. 



The gill structure in the male is in both gills the same as described 

 for the outer gill in Hyria. In the sterile female (pi. VII, fig. 7) it is 

 as in the female of Hyria. Also here the marsupial part does not 

 occupy the whole gill, but only a portion in the middle, leaving a 

 larger portion free anteriorly and a smaller portion posteriorly. 



Thus Tetraplodon is distinguished from Hyria not only by the 

 shape of the shell, but also by some differences in the soft parts: the 

 chief is the connection of the mantle edges in front of the branchial 

 opening. Whether this is found in all species remains to be seen. 



* I do not understand why Simpson (Pr. U. S. Mus. 22, '00, p. 866) places 

 this species with Caalalina v. Iher., since it is undoubtedly a Castalia (= Tet- 

 raplodon); von Thering himself placed this species correctly. 



