278 Annals of the Carnegie Museum. 



Subfamily ANODOSTINJE. 



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. 23 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 NVIII, 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 maisupium, 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 Anodontina are known from countries without a winter, but such are 

 present. It would be very interesting to study their behavior in this respect. 



