THE NAUTILUS. 19 



Sometimes mature, or apparently mature glochidia and quite un- 

 developed ova are found mixed up in the branchial sacks. Whether 

 the latter will develop into embryones later, or remained unchanged 

 for want of impregnation, remains to be studied up. 



2. Branchial sacks, or uteri. On Lampsilis, the branchial sacks 

 are differentiated even when not charged with ova or young. They 

 are situated in the posterior part of the outer branchia, in a group, 

 the marsupium, which, when charged, is very considerably enlarged, 

 often exceeding half the length of the shell, and crowding away the 

 unchanged anterior and posterior parts of the branchiae. It has 

 already been said that the number of sacks is, to a certain degree, 

 characteristic for each species, yet rather variable even in individ- 

 uals of the same size, and it is also hardly ever the same on the two 

 sides. In the young, there are only a few, and their number is 

 increasing with the age of the auimal. They are also not all of the 

 same size, and each one may occupy a smaller or greater number of 

 branchial filaments. 



In younger animals, there are always a number of small, empty 

 sacks adjacent to the gravid ones, preformed to be charged in the 

 following year. 



The shape of the uterus sacks in U. irroratus Lea is known from 

 the author's description and figure. There is considerable variation 

 in their numbers. Of three specimens from the same place, all 

 medium sized, one had seven sacks on one side, four on the other, 

 the second had eleven and ten, the third, ten and eight. At the 

 proximal ends there were exclusively ova ; at some distance, those 

 in the periphery had transformed into glochidia, and at the distal 

 ends the latter were in excess, while a great number of ova had still 

 remained unchanged. In accordance with this, the flesh color was 

 much more intense at the proximal than at the distal ends, as the 

 ova are colored, the young colorless. 3 The ova are packed closely 

 together and coherent by some intermediate substance, so that the 

 whole worm-like cylinder can be extracted in toto from the enclos- 

 ing membrane. 



The young, in the uterus, show marked differences from those of 

 all other species seen, as to soft parts and shell. The latter is con- 

 siderably longer than high and has numerous distinct, crowded, con- 

 centric lines of growth. Its length is O21, alt. 0'17, diam. 0'14 mill- 

 imeters. 



* In one specimen, the ova, and so the whole cylinders, were colorless, a rare 

 exception. 



