Il8 BULLETIN OF THE BUREAU OF FISHERIES. 



Quite recently Ortmann (1910a, 191 1) has proposed an entirely different arrange- 

 ment cf the Naiades which is based upon a study of the anatomy and the larval charac- 

 ters of the fresh-water mussels of Pennsylvania. His system also lays especial stress 

 on the marsupial differentiations, but it involves a number of important modifications 

 in vSimpson's classification which he maintains must be radically recast, in the light of 

 the facts which he has discovered, if it is to represent the natural affinities of the group. 



It is not our purpose to present a critical discussion of the relative merits of the 

 two systems, as our only interest in this connection is concerned with the marsupium 

 as an accessory organ of reproduction, but as Ortmann has added a number of important 

 facts to our knowledge of this structure, it is necessary to state briefly the basis of his 

 classification so far as it has to do with the several marsupial modifications. In addi- 

 tion to the marsupial structure, he makes use in his arrangement of families, subfamilies, 

 and genera of a number of other characters which he considers of systematic value; 

 for example, the degree of fusion of the inner lamella of the inner gill with the visceral 

 mass; the dorsal aperture (supra-anal opening); the siphons; the differentiations of the 

 mantle edge; the structure of the glochidium; and shell characters. In contrasting his 

 arrangement with that of Simpson, however, reference will be made only to the 

 marsupium. 



Confining himself to North American forms, he divides the Naiades into two 

 families, the Margaritanidse and the Unionidte. His discovery that in Margaritana 

 margaritifera there are no distinct interlamellar junctions in the gills, but only scattered 

 interlamellar connections, and consequently no definite water tubes, he considers of 

 sufficient importance to warrant him in creating a new family for this genus, Margari- 

 tanidae, which he has thus sharply set apart from the remaining genera grouped under 

 the Unionidae, a procedure of doubtful wisdom." The fact that complete interlamellar 

 junctions are absent in Margaritana, which is further characterized by certain other 

 apparently primitive features, is of the greatest interest, but that these differences are 

 of sufficient significance to justify a separate family for Margaritana is not at all clear. 



The Unionidae, after the removal of Margaritana, he divides into three subfamilies, 

 distinguished as seen below by definite marsupial characters: 



I. Unioninse. "Marsupium formed by all four gills, or by the outer gills only; 

 edge of marsupium always sharp and not distending; water tubes not divided in the 

 gravid female." 



This subfamily includes the following genera, which, however, he has recast to a 

 considerable extent by subtractions and additions of species: Quadrula Rafinesque 

 (including Tritogonia tuberculata) ; Roiundaria Rafinesque (established for Quadnda 

 tubercidata) ; Pleurobema Rafinesque (including Q. coccinea, pyramidata, ohliquu, cooperi- 



o The condition described by Ortmann for M argaritana is quite similar to that which is found in the gills of Mytitus Ccf. 

 Peck, 1S77), in which complete interlamellar junctions are absent and the inner and outer lamellae are connected oidy by scattered 

 strands of subfilamentar tissue passing across the interlamellar space. This similarity in gill structure would argue strongly 

 for the primitive position of Margaritana among the Unionidae. In Lucina these interfilamentar junctions are larger and are 

 provided with blood vessels, while in Myiilus they are non-vascular. Ortmann does not state whether or not they contain blood 

 vessels in Alargaritana. 



