ortmann: south American naiades. 457 



of the North American Elliptio, that without a minute examination of muscular 

 impressions, hinge-teeth, etc., it is imiwssible to recognize them, when the locality 

 is unknown. The only reliable character in this case, beak-sculpture, is often 

 obliterated by erosion. Species of Anodontites often look like species of Anodonla. 

 Here the examination of beak-sculpture, muscular impressions, apd the ligamentinal 

 sinus establishes their affinity. On the other hand, there are Mutelidce which are 

 easily I'ccognized by the peculiar shape of the shell, and could never be confounded 

 with Uniomdce {Castalia, Hyria, Mycetopoda, etc.). 



The characters of the two subfamilies of the Mutelidce are the following: 



Subfamily Hyriin.e Ortmann (1911). 



Hyriance Swainson (1840), very closely corresponds to this. Lamphornmphus- 

 group of Hyriance Simpson (1900). 



1. Anal opening (Figs. 1, 2, a) closed above (s), slit-like (in South American 

 forms), or forming a short, tubular siphon (Australian forms). 



2. INIarsupium generall}' an interrupted network of interlaminar connections 

 (Figs. 1, 2, i), the connections often standing in rows, thus forming incomplete 

 septa and incomplete, communicating, water-tubes. In rare cases, the inter- 

 laminar connections of the marsupium (but not of the non-marsupial gills) form 

 solid septa and isolated water-tubes. Marsupial part of gill often restricted to 

 only a section of it. 



3. Non-marsupial gills always with poorly developed, scattered interlaminar 

 connections. (Figs. 1, 2, o.) 



4. Inner lamina of inner gills always entirely connected with abdominal sac. 



5. Palpi (Figs. 1, 2, h) subtriangular or subfalciform, with gently curved lower 

 margins, and somewhat produced posterior points. 



6. Larva a glochidium (See Fig. 4, p. 469). 



As a character of the shell may be mentioned the beak-sculpture, which is 

 generally present (often jioorly developed, rarely absent), and always radial. 

 The hinge-teeth are always present, and generally well-developed. Dorsal muscle- 

 scars are present. 



Subfamily Mutelin^e Ortmann (1911). 



Mutelidce Adams (1S58), Simpson (1900). The name depends, of course, on the 

 genus Mutcla, the anatomy of the soft parts of which, as has been pointed out 

 already, remains to be investigated. 



1. Anal opening (Fig. 3, a) open (in South American forms, except Myceto- 

 poda) or closed above (in African forms and Mycetopoda), slit-like. 



