THE NAUTILUS. 108 



course their existence in otlser Australian species should be con- 

 firmed. For the present, I think, it is well to separate the Australian 

 form generically from the South American Diplodon, or, in other 

 •words, we should give to the subgenus Hyridella Swainson, 1840, 

 generic rank. This is supported by the fact that Hyridella also 

 differs in certain shell characters from the typical Diplodon, as has 

 been recognized already by Simpson (1. c. p. 888). 



One very important conclusion, however, is now finally estab- 

 lished : Simpson's opinion that the Najades of the type of Diplodon 

 (^Hyridellu) australis are closely related to certain South American 

 forms (typical Diplodon), is fully justified, and there remains not the 

 slightest doubt about this. The structure of the soft parts of both 

 groups is so similar and so greatly different from the true Unionidae 

 of the rest of the world, that Hyridella, no matter whether we regard 

 it as a genus or a subgenus, must be placed with the family 

 Mutelidne (?) and the subfamily Hyriinae (see Nautilus 24, March, 

 1911, pp. 129, 130). This affinity is of the utmost zoogeographical 

 importance. 



MUSCTILIUM DECLIVE, N. SP. 



BY V. STERKI. 



Mussel rather small, subequipartite, slightly to moderately in- 

 flated ; beaks not or little anterior, somewhat prominent over the 

 valve margin ; the latter, anteriorly and posteriorly, straight or 

 slightly curved, forming the two shanks of a rounded angle between 

 the beaks, of about 130°, the posterior incline placed higher up than 

 the anterior; balance of the outlines rounded without any angles in 

 full-grown specimens ; in half-grown and adolescent there is a short 

 truncation at the posterior margin, at right angles to the longitudinal 

 axis, and a similar one at the anterior, somewhat oblique ; in young 

 — post-nepionic — specimens, the posterior part of the mussel is 

 shorter and higher than the anterior ; surface glossy to w'axy, with 

 very fine (microscopic), sharp, crowded, concentric strije, and 

 usually one or two lines of growth, and faint, irregular radial mark- 

 ings ; shell thin, transparent to translucent ; color light amber, to 

 somewhat grayish or brownish in old specimens; hinge rather long; 



