456 



MEMOIRS OF THE CARNEGIE MUSEUM. 



4. Gills with or without water-tubes, and with isolated, scattered inter- 

 laminar connections, or with interrupted or solid septa. (In the Unionidm, mostly 

 uninterrupted septa and water-tubes are found; very rarely are they interrupted; 

 the forms with the latter probably form the connection with the Mutelidce.) 



5. Marsupium only in the inner gill (Figs. 1, 2, 3, i). (In the Lhiionidce, the 

 marsupium is either in all four gills, or in the outer gills, never in the inner gills 

 alone.) 



6. Certain advanced genera of the Mutelidce show a tendency to close the 

 branchial opening in front by a mantle connection. (Such a connection is entirely 

 unknown in the Unionidce.) 



The foregoing are the anatomical characters. The shape of the shell and its 

 parts are subject to so many variations, "both in the Mutelidce and the Unionidce, 

 that it is practically impossible to point out general differentiating characters. 

 Different and often very peculiar types of shell, occurring independently, are fre- 

 quently observed, so that it is clear that the shells by responding to certain stimuli 



and requirements have acquired sim- 

 ilar shapes in forms, which belong to 

 different families (convergency or 

 parallelism). Simpson (1900 and 

 1914) found it impossible to give 

 shell-characters for his subfamily 

 " Hyriance" = Hyriince-\-certSim Uni- 

 onince, except beak-sculpture, and he 

 was mistaken in this. In our arrange- 

 ment, this is the only character, 

 which might be mentioned, but not 

 without qualification. We may ex- 

 press it thus: The Mutelidce have ra- 

 dial beak-sculpture, if such is present 

 at all);''' while the Unionidce rarely 

 have distinct radial beak-sculpture, 

 but commonly other types, zig-zag, double-looped, or concentric. 



Some Mutelida; have, indeed, shells which completely mimic those of certain 

 Unionidce. Species of Diplodon often externally so much resemble certain species 



'But even thi.s .should be (lualifiod. In certain species of Aiwdontites in the subfamily Muteliiw^ 

 I have observed something like concentric beak-sculpture, while, as a rule, the Mutelina; have no beak- 

 sculpture whatever. 



Fig. 2. Diagram of the soft parts of female of 

 Cadalina nchringi Von Ihering. Natural Size, left 

 section of mantle removed. (Lettering as in Te.xt- 

 figure 1, p. 4.55.) 



