ortmann: south American naiades. 567 



larval form is a Idsidiiiin (Size: 0.10 mm). It i.s a very singular circumstance 

 that I have not been able to find lasidia (or any other form of mature larvae) in 

 my material, although a good many gravid females of various species and genera 

 are at hand. 



The Genera of the South American ]\Iutelin^. 



• 



Mutelince are found in South America and in Africa. Unfortunately, among the 

 African forms, the anatomy of Spatha (including the subgenus Aspathnria) is 

 alone known (Ortmann, 1910, p. 39; 1918, p. 75); but this genus differs from most 

 South American Mulelince (Fossula, Monocondylcea, Anodonlites) by the fact that 

 the anal opening is closed above, without forming a supra-anal opening. In the 

 South American genera this opening is open and not at all closed, with one exception, 

 Mycetopoda, where about the upper half of the anal is closed. In addition, in 

 Spatha, the inner gill is free from the abdominal sac, while in the South American 

 genera it is connected with it. All other characters are similar. Thus, although 

 closely allied to Spatha, the South American genera form a group by themselves, 

 and the similarity of Mycetopoda to Spatha in the anal opening apparently indicates 

 only parallelism of development, not genetic relationship. 



It is hard to say which group is more primitive, since of the two differing 

 characters, the one (anal opening) is more primitive in the American forms, the 

 other (inner lamina of inner gill) more primitive in the African Spatha. The 

 latter again is rather advanced in the shell, having no hinge-teeth, a condition 

 which is also found in most South American Mutelins, but not in all, for Iherinyella, 

 Fossula, and Monocondylcea have at least pseudocardinals. 



There is no question that among the South American forms, these genera with 

 hinge-teeth should be regarded as more primitive. But a confirmation of this 

 cannot be found in the anatomy, the latter being alike in all of them. Possibly 

 the anatomy of Iheringella might furnish some enhghtenment , l)ut of this genus 

 the soft parts have never been observed. 



The structure of the hinge might be expected to furnish evidence of the con- 

 nection of the Mulelina; with the Hyriinoe. The very fact that there are genera 

 among the Mutelina' with hinge-teeth, indicates that the typical South American 

 forms (of the Anodontites-type) are derived from forms with hinge-teeth. But 

 the structure of these hinge-teeth is rather peculiar. They are always in a rudi- 

 mentary condition, the laterals being missing, and the other teeth corresponding 

 to the pseudocardinals cannot be positiv(>ly homologized with the pseudocardinals 

 of the Hyriirm, Yet we are to assume on account of the many anatomical points 



