REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. 135 



distended into rather conspicuous bulb-like expansions which greatly diminish the 

 openings of the tubes into the suprabranchial chamber, although their edges do not 

 fuse. 



As the histological details of the structure of the raarsupia in several genera belong- 

 ing to the Lampsilinae have been studied by Mr. Carter and will be described in his 

 forthcoming paper, a further account may be omitted here. 



PHYLOGENY OF THE MARSUPIUM. 



It is not without justification that a phylogenetic significance should have been 

 attached to the several types of the marsupium which occur in the Unionida;, for it would 

 seem clear that those forms in which the structure characteristic of the respiratory 

 gill is least modified, as in Quadrula, are more primitive than those in which the special- 

 ization of the marsupium has gone much farther, as in Anodonta, LampsUis, and many 

 other genera. 



Simpson (1900) has considered these facts in some detail and concludes that the 

 oldest type of marsupium phylogenetically is that occurring in the Endobranchiae in 

 which the inner gills alone are used as brood chambers. It is a sUght transition from 

 this condition to that presented by the Tetragenae with all four gills functioning for this 

 purpose. Basing his supposition largely upon shell characters and geographical distri- 

 bution, he further concludes that the Homogense marked the next step in marsupial 

 differentiation, while the Heterogenae and all other groups in which a portion only of the 

 outer gills is modified for receiving the eggs are the latest product of the evolution of 

 the Unionidse. 



That this series correctly represents the phylogenetic sequence in the appearance of 

 the marsupial modifications would seem to be borne out by the structural conditions 

 existing in the several types so far as we have examined them, provided that we assume, 

 with respect to the Homogenae, that genera like Pleurohema and Unio, in which the mar- 

 supium is less specialized, are more primitive and therefore stand nearer the TdrageruB 

 than such genera as Anodonta, Syviphynota, and others, which, as Ortmann has shown, 

 exhibit certain modifications evidently in advance over the marsupium of the former. 



Ortmann (191 1), although he does not consider the Endobranchiae, has arrived at con- 

 clusions essentially similar to the above. He points out, however, that the absence of 

 complete interlamellar junctions in the gills of Margaritana would indicate that the new 

 family which he has created for this genus, Margaritanidae, is the most primitive group 

 of the Naiades, and this inference, as was indicated above, is further strengthened by the 

 fact that the simple gill structure of Margaritana is apparently similar to that of Mytilus, 

 which belongs to a lower group of lamellibranchs than the fresh-water mussels. 



His conclusions concerning the sequence of his three subfamilies of the Unionidae may 

 be quoted (p. 328) : 



Of the UnionidcE, the Unionina: are certainly more primitive than the other two subfamilies, as is 

 evidenced by the simple character of the structure of the marsupial gills. The Anodontina and Lamp- 



