124 BULLETIN OF THE BUREAU OF FISHERIES. 



are turned slightly inward toward the median plane. The marsupium is well shown in 

 figure 8, plate vii. The distension of the marsupial water tubes begins at quite a distance 

 above the ventral border of the rest of the gill, as is seen in the figure. The anterior 

 respiratory portion is sharply separated from the rest of the gill by a cleft which extends 

 almost up to the level of the suprabranchial chamber. At first this -was supposed to 

 be an artificial split, but as it occurs on both sides and its edges are perfectly smooth 

 and show no indication of injury, we have concluded that it must be a normal condition. 

 Unfortunately we have had no other specimens with which to compare it. 



In our specimen the marsupium is slightly tinged with pink, the color being due to 

 unfertilized pigmented eggs which are scattered among the glochidia. Simpson speaks 

 of the marsupium as being purple. 



The unusual form of the marsupium in Cyprogenia was originally described by 

 Lea (1827) in C. irrorata, but curiously enough he reversed the direction of the coil 

 in his figure, which appears to have been drawn from memory, as such a mistake could 

 hardly have been possible if he had had a specimen before him." 



Call (1887) many years later described a similar marsupium in C. aberli Conrad, 

 which he very crudely figured. It is strange that, although he reproduces Lea's original 

 figure of irrorata by the side of his own, he makes no mention of the error in it. Judging 

 from Call's figure, the number of tubes in the marsupium of aberti is much larger than in 

 irrorata. He shows about 20, while Lea states that there are 7 or 8 in the latter, and 

 in our specimen there are 7. Simpson gives the number for the genus as 7-23. 



PtychogencB. — This group contains a single genus, Ptychobranchiis. The marsupium 

 occupies the lower half of the entire outer gill and is thrown into a series of folds, from 

 6 to 20 in number, according to Simpson. Each water tube of the marsupium is inflated 

 at its distal extremity to form a globular enlargement projecting beyond the interla- 

 mellar junctions — a condition which gives to the free edge of the gill the beaded appear- 

 ance so characteristic of the genus. This marsupium is well illustrated in figure i, 

 plate VI, which is drawn from a gravid female of P. phaseolus Hildreth. Seventeen 

 conspicuous folds, sharply demarked from each other, are shown in the figure, in which 

 the beaded border of the gill is also clearly seen. 



Eschatigenm. — Simpson has established this group to receive the genus Dromus 

 in which the marsupium occupies the ventral half of the outer gill throughout the greater 

 portion of its length. We are indebted to Dr. R. E. Coker for several specimens of 

 Dromus dromus Lea, obtained from the Cumberland River in Kentucky in November, 

 1 910, which have furnished the material for our study of this type of marsupium. Three 

 gravid females, all containing glochidia, were included in the lot. 



As seen in figure 4, plate vii, the line of demarcation between the dorsal respiratory 

 portion and the ventral marsupial region is quite sharp and regular, owing to a constric- 

 tion of the gill where the two regions join. Below this line the gill is swollen to an extent 

 varying with the degree to which it is charged with glochidia. The anterior end of the 

 gill is not included in the marsupium and is sharply folded over on the outside of the 



o We are indebted to Mr. Bryant Walker, of Detroit, for having called our attention to this error in Lea's figure. 



