118 PKOCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



papilloc will meet and fuse with tlicir neiglibours on the opposite 

 lamella, forming the scattered syuapticulae which hold the gill-plates 

 in shape. Otlier simple features such as the incomplete diapliragm, 

 undeveloped siphons, indiscriminate use of all the gills as a rudimentary 

 marsupium, and the little modified subglobular-quadrate glochidia, 

 may be noted. 



From this simple type differentiation has proceeded in at least two 

 directions, one marked by the southern hemisphere groups, apparently 

 represented in their simplest condition by Uyr'ut (Hyriinae), while 

 Spatha (Mutelinse ?) is a more specialized phase, the otlier embracing 

 the numerous and diverse genera which occur in the Noitliern 

 Hemisphere. The simplest aspect and the one least modified from the 

 fundamental Margaritanoid structure among the latter is exemplified 

 in Quadrula (Quadrulina3), in wliich the interlamellar gill-connexions 

 are completely coalesced into definite water tubes, the diaphragm fully 

 developed, formed solely by the gills, while on the other hand all the 

 branchiae are retained as a simple marsupium and the glochidia remain 

 undifferentiated. This group has apparently given rise to another 

 closely allied one, in which the brood-pouch has been restricted to the 

 outer gills only. Sevei'al American genera, FIcurobema, Rohmdaria, 

 Elliptio, etc., belong to this division, which may be termed the 

 Pleurobeminas. From the Pleurobeminse in turn no less than two 

 stocks have arisen. One of these is represented by Lampsilis 

 (Lampsilinre), in which a portion of the outer gills in the female 

 are permanently modified to serve as a highly organized marsupium 

 but in which the glochidia remain simple. From this tj-pe Proptera 

 (Propterinfc) has been derived, distinguished by its peculiarly modified 

 * axe-head' larvae, thus representing the culmination of the evolutionary 

 cycle in this branch of the Naiads. Unio (Uniouinse) represents tiie 

 other stock derived from the Pleurobeminas. Here the glochidia have 

 advanced in specialization, becoming triangular and armed, resembling 

 a spade-head in shape, while the simple marsupium of Pleurohema is 

 retained. Anodo7ita is a specialization of this type in which the outer 

 gills of the female become transformed into a temporary highly 

 modified marsupium during the gravid season, having advanced pari 

 passu with the modification of the glochidia, thus resulting in 

 a condition analogous to that of Spatha and Proptera. So much for 

 the internal structure; the problem now confronts the systematist as 

 to how all this may be best classified. 



A species in the Hyattian sense consists of a group of related 

 individuals having a similar genetic history and possessing a tendency 

 to evolve along strictly analogous lines. If this group may be divided 

 into sub-groups which occupy a particular stage in the development- 

 series, then they may be termed sub-species,' while a group of species 



' This definition, while very diilei-ent from that ordinarily given for a sub- 

 species (that it represents a geographic variety), in practice amounts to 

 approximately the same thing. Sub-species either occupy a different 

 geographic area, a different station, or a different life-zone from the type, 

 so far as the writer's experience goes. 



