THE NAUTILUS. 129 



complete and heavy septa separate completely isolated water tubes. 

 In none of these cases the structure of the inarsu[)ial gill is identical 

 with that of the Unionidse, although the septa and water tubes in the 

 latter case show certain analogies to those of the Unionidse. How- 

 ever, the finer 8tru(;ture being different, I consider this a case of 

 convergency of structure. 



(7) Of these anatomical differences representing functional differ- 

 entiation, we may say that the oldest must be the formation of llx- 

 diaphragm, for the tendency to form siphons necessarily must start 

 with it. Almost as old as this is the differentiation of the marsupium, 

 and thus the location of the latter and its structure must be essential. 

 To these characters we consecjuently should attribute the greatest 

 systematic value, and thus it is evident that the genus Margaritanu, 

 which I have placed formerly (Nautilus 23, Feb., '09, p. 116) in 

 a subfamily by itself, but within the family Unionidiz, should rank 

 higher, and I do not hesitate now to call it a family. Tims we would 

 have the following three families among the Naiades: 



Family : MARGARiTANiDiE. Diaphragm incomplete, formed 

 only by tlie gills. Anterior end of inner gills distant from the 

 palpi. Branchial and anal siphons ill defined, and not closed above, 

 and no supra-anal developed. Gills without water tubes and with 

 irregularly scattered interlamellar connections. Marsupium and 

 glochidia unknown. 



Family : Unionidse. Diaphragm complete, formed only by tlie 

 gills. Anterior end of inner gills distant from the palpi. Branchial 

 and anal siphons sharply separated from one another by the dia- 

 phragm. Anal very rarely not closed above, generally closed, but 

 with a supra-anal opening (which very rarely may be obliterated). 

 Gills with water tubes and distinct, continuous, interlamellar septa. 

 Marsupium in all four, or only in the outer gills. Larva a glo- 

 chidium. (Contains three subfamilies : Unioninee, Anodontince and 

 Lampsilinae. ) 



Family : MuTELiDiE.' Diaphragm complete, formed anteriorly 

 by the gills, posteriorly by the mantle. Anterior end of inner gills 

 in contact with the palpi. Branchial and anal siphons sharply sep- 

 arated from one another. Anal open or closed above, but there is 

 never a supra-anal opening. Gills with very indistinct and inter- 

 communicating water tubes or with well-developed water tubes. 

 Marsupium only in the inner gills. 



