Ortmann: Families \m> Genera of Naiades. 



:) 1 7 



lies in the outbulging pari of the gill, and only the basal ends extend 

 very little in between the original gill-laminae. 40 



In young specimens, the marsupium is simple, and forms a smooth, 

 compressed (not much swollen) body, marked off from the anterior 

 and posterior non-marsupial parts of the gill by irregular folds. In 

 larger specimens, however, this marsupial mass begins to warp, and 

 finally is folded up into a number of irregular folds. The strongest 

 folds arc near the anterior end of the marsupium. In none of my 

 specimens docs the marsupium occupy the whole margin of the gill. 



The placenta? are quite solid and 

 permanent, and possess a peculiar 

 structure (see Plate XVIII, fig. n). 

 In all specimens at hand glochidia are 

 developed, and they appear arranged 

 around a central axis (placenta), the 

 color of which is white or red. The 

 latter color, if present, is restricted to 

 this axis, and the glochidia themselves 

 are transparent white, and form a 

 fringe around the narrow edges of the 

 placentae. They seem to be connected 

 with them by fine threads, possibly 

 their embryonal threads. 41 Since the 

 placentas themselves almost touch the 



septa, the glochidia are thus crowded toward the lateral faces of the 

 marsupium. Whether this arrangement is already present in the 

 eggs, is unknown to me. It is, however, certain that this arrangement 

 can only have the purpose of bringing the glochidia as close as pos- 

 sible to the wall of the. marsupium, in order to give them the best 

 chance to be near the current of fresh water going over the mar- 

 supium. This is one of the little special devices for the proper 

 aeration of the glochidia. 



Glochidia of unique shape; they are much longer than high, and 



40 Simpson says that the marsupium occupies the "base" of the outer gills. This 

 i- a very ambiguous expression, but apparently is intended to imply that it is 

 -ituated on the margin of the gills, while the "base" is non-marsupial. This is a 

 very peculiar feature of Dromus, and not met with in any other genus. Only 

 l'tychobranchus has the same condition slightly indicated. 



41 The glochidia adhere rather firmly to the placental mass by their threads 

 and it is hard to isolate them, except with caustic potash. 



mp 



Fig. i 86. Left gills of a large 

 gravid female (shell 6o mm. long), 

 from Cumberland River, near Ro- 

 wena, Russell Co., Ky. (Cam. 

 Mus., Xo. 6i, 4.966.) 



