ortmann: south American naiades. 461 



The differences of the South American genera may be tabulated as follows: 



a. Shell nut alatc, or diily very sliKlitiyahitc hcliiiid, suhclliptifal, subovati', suhrotiiiul, or subtriangular. 

 6. Shell subclliptical, subovate, or siibrdtund, but not subtriangular. Beaks not much elevated, 

 with radial beak-sculpture, variously developed. Posterior ridge absent or poorly developed; 

 when present and distinct, the shell is elongated. Hinge teeth more or less smooth or dis- 

 sected, or corrugated, but without vertical parallel ridges, luterdentuni very narrow. 



Diplndon. 

 bb. Shell subtriangular (or subciuailrate). Beaks more or less elevated and with rather deep beak- 

 cavities. With or without beak-sculpture. Posterior ridge well developed, defining a 

 distinct posterior slope. Hinge-teeth generally nnicli dissected, and often with [Kirallel 

 vertical ridges. Interdentum well developed, rather broad. 

 c. Sides of shell somewhat flattened, posterior ridge moderate, posterior slope elevated in the 

 middle and slightly alate (thus the shell becomes subalate). Hinge-teeth smooth, crenu' 



lated, or with parallel ridges Cadalina. 



cc. Sides of shell more or less convex, posterior ridge sharp, posterior slope truncated, not 

 elevated (or only so in the young). Hinge-teeth with parallel ridges. 

 d. Radial beak-sculpture present. 



e. Beak-sculpture strongly developed, extending over a kirge part of the disk. Surface 



not concentrically sulcated Caslalia. 



ee. Beak-sculpture short anteriorly, extending farther on the disk posteriorly. Surface 



covered with strong, even, concentric ridges (sulcated) Caslaliella. 



fid. Beaks without tlistinct sculpture, and surface of shell smooth Callonnia- 



aa. Shell subrhomboidal or subtriangular, strongly alate behind, slightly alate in front. 



b. Beaks with strong radial sculpture. Posterior ridge moderately developed Hijria. 



bb. Beaks without sculpture. Posterior ridge strongly developed, sharp and high I'risodon. 



From the expressions used in distinguishing these genera, it appears that 

 Diplodon is a rather composite genus, containing shells of various, more or less 

 indifferent types, while the other genera represent more marked and i)cculiar 

 shapes, with Castalina forming, to a degree, a connection between Diplodon and 

 the rest. This indicates that Diplodon includes the more primitive and generalized 

 forms of the subfamilj^, while the others are more specialized and advanced. In 

 a general way, this is supported by the investigation of the anatomy. However, 

 it is much to be regretted that I am imable to give any information as to the struc- 

 ture of the soft parts of the genera Caslaliella and Callonaia. The former is en- 

 tirely unknown to me, and of the latter I jjossess only two odd valves. Also of 

 Prisodon I only have the soft parts of a young individual. Better material is at 

 hand to represent the other genera. 



Generally speaking, the anatomy of all these forms is rather uniform; yet 

 there are certain characters in which they differ, and some of these, indeed, indi- 

 cate a gradual advance in the structure. 



