REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. 1 23 



When the marsupium is less heavily charged, as in young females, the ventral 

 expansion may not be great enough to cause the conspicuous fold just described, and in 

 cases like this the marsupium, which may then appear kidney-shaped, is marked off 

 from the respiratory end merely by a notch by reason of its greater depth. Such a 

 case is seen in figure 6, plate vii which is taken from a gravid female of L. recta. 



Simpson has included 14 genera in the Heterogenae, only three of which, however, 

 have come under our observation, namely, Lampsilis (including Proptera), Obovaria, 

 and Plagiola. We have recorded this type of marsupium in Lampsilis alata Say, anodon- 

 ioides Lea, gracilis Barnes, higginsii Lea, IcEvissima Lea, ligamentina Lamarck, luieola 

 Lamarck, recta Lamarck, subrostrata Say, and ventricosa Barnes; in Obovaria ellipsis 

 Lea; and in Plagiola elegans Lea and securis Lea. 



No case of pigmented eggs has been encountered by us in this group, and unferti- 

 lized eggs in the marsupium are exceedingly rare. 



Mesogenct. — This group is so designated by Simpson to include the genera Cypro- 

 genia and Obliquaria, in which a variable number of enlarged water tubes in the middle 

 region of the outer gill are specialized as the marsupium, a larger anterior and a shorter 

 posterior portion of the gill retaining the ordinary respiratory character. We have 

 studied the condition in Obliquaria rcflexa Rafinesque and also in Cyprogenia irrorata 

 Lea, in which the structure of the marsupium is essentially the same, although the two 

 cases differ strikingly in general appearance. 



The marsupium of Obliquaria reflexa is shown in figure 7, plate vii. Here the modi- 

 fied water tubes, which project far down below the border of the rest of the gill, appear 

 enormously swollen when gravid and show a tendency to curve backward, the degree 

 of curvature becoming progressively greater in the tubes from the anterior to the pos- 

 terior end of the marsupium. A gradual decrease in the length of the tubes takes place 

 in the same direction. The tubes are slightly larger at their distal ends, so that their 

 form is somewhat club-shaped; this is seen more clearly in the shape of the egg masses 

 which form perfect casts of the cavities of the tubes (fig. 42, pi. xi). The corrugation 

 of the lower border of the marsupium is very conspicuous in the figure. The number 

 of water tubes comprising the marsupium in this species is not at all constant, but on 

 the contrary varies in the individuals examined by us from two to eight ; according to 

 Simpson, they range from four to seven. During the breeding season each tube is entirely 

 filled with embryos or glochidia which adhere so firmly together that they form a mass 

 of tenacious consistency. 



In Cyprogenia, the only other genus included in the group, the marsupium may 

 be regarded as a further development of the condition seen in Obliquaria. We have 

 observed it in but a single individual of C. irrorata, which was kindly sent to us by Dr. 

 R. E. Coker. This specimen, which contained fully formed glochidia, was collected 

 in the Cumberland River, Kentucky, in November, 1910. The tubes of the marsupium, 

 which present a most striking and unusual appearance, spring from near the middle 

 of the outer gill, are enormously elongated, and curved backward into a close coil, a 

 part of the coil passing under the posterior unmodified portion of the gill, as the tubes 



