278 ANNALS OF THE CARNEGIE MUSEUM. 
Subfamily A NODONTINA. 
To this subfamily belongs, first of all, the European genus Ano- 
donta, which is the typical genus, and which also occurs in North 
America, and probably likewise in Asia. In North America there are 
a number of additional genera, in some respects even more primitive 
than Anodonta, of which I have examined the following: Alasmidonta, 
Strophitus, Symphynota, Arcidens, Anodontoides, Lastena. They are 
all adopted from Simpson’s Synopsis, and I do not see any reason for 
changing these generic divisions. 
In the soft parts, they all very closely resemble each other. The 
fundamental idea, the physiological meaning of the anatomical pecu- 
liarities of this group, which governs its structure, is the following: 
these forms are bradytictic, and the breeding season becomes a long 
one, and the glochidia, after having fully developed, are not discharged, 
but kept in the marsupium over winter.** This makes necessary a 
special apparatus for supplying the glochidia with the necessary oxygen 
during this period. The problem is solved by the development of a 
special apparatus to secure the circulation of water within the gills, 
which, in the diagnosis (p. 224), has been called that of the “lateral 
water-tubes.”’ This apparatus exists only during the breeding 
season, but it has been found in all species the gravid females of 
which have been investigated. In sterile females traces of it are also 
generally discernible, since the lateral parts of the water-tubes often 
show indications of its presence in the conformation of their epithelium 
(see Plate XVIII, fig. 6). This is the most essential character of the 
subfamily. 
Other characters are furnished by the development of thickened 
tissue along the edge of the marsupium, which permits the distending 
of this gill during pregnancy, and this character is also generally 
easily seen in sterile females. Further, the mantle-connection sep- 
arating the anal and supra-anal is generally well, often very well, 
developed; the inner gill has the inner lamina free or connected with 
the abdominal sac. These latter two characters are of secondary 
value, but they help somewhat in the distinction of genera. The 
marsupium is always formed by the outer gills, the glochidia are rather 
large, subtriangular, and possess hooks. There are no generic distinc- 
tions observable in these characters, although the shape and size of 
23 Very few Anodontine are known from countries without a winter, but such are 
present. It would be very interesting to study their behavior in this respect. 
