324 MEMOIRS OF THE CARNEGIE MUSEUM 
mantle, which is rather short, and leaves a supra-anal of considerable length; 
branchial opening rather well defined anteriorly; no papille or special struc- 
tures on the edge of the mantle in front of the branchial opening; inner lamina of 
inner gills free from the abdominal sac; elochidia imbedded in rather well 
developed placentz, or placents poorly developed; glochidia small or of medium 
size, semicircular or semielliptical, with rounded ventral margin, and without spines. 
The discharge of the glochidia is by the natural channels through the anal opening. 
These forms are tachytictic. 
With regard to the characters of the shell little can be said, for the shape of 
the shell is very variable. Generally the shell in this eroup is rather heavy, from 
more or less rounded to more or less elongate. The colors of the epidermis are 
dull or brighter, rarely with bright markings (rays); sometimes peculiar, odd shapes 
of the shell are met. The sculpture of the beak is variable, concentric, or with indi- 
cations of double loops, or pustulous, or even with traces of zig-zag sculpture toward 
the disc. Very often the sculpture of the beak is obsolete; hinge always complete, 
with well developed teeth, often quite heavy; male and female shell in most cases 
absolutely undistinguishable. 
Remarks: It is practically impossible to recognize this group by shell char- 
acters, although there are certain peculiar types of shell, which are found in this 
group alone. Simpson failed entirely to recognize the close affinity of the genera 
belonging to this group, and places them in three different divisions of his system, 
bringing his Unio and Pleurobema together with the Anodonta-group, with which 
there is no close relationship whatever, and removing Tritogonia entirely from 
them. The genera of Simpson’s system belonging here cannot be maintained as 
defined by Simpson, and material changes will be necessary as will be shown here- 
after. 
2. The second group comprises the following genera of Simpson: Strophitus, 
Anodonta, Anodontoides, Symphynota, and Alasmidonta. 
Here each ovisac of the gravid female is not formed by a whole water-tube, 
but only by a part of it, the middle one, which is separated from two lateral canals 
by a folding up of the epithelium of the septa. In addition, the ovisacs are closed 
above at the base of the marsupial gill, thus forming a completely closed sac within 
each water-tube. In one case (Strophitus) this sac is again divided into secondary 
compartments. The edge of the marsupial gill becomes adapted to the swelling 
of the gill in the breeding season, and gives way, the two lamin drawing apart; 
l’[his holds good only for the Pennsylvanian species examined by me. In exotic forms belonging to this group, the 
inner lamina may be connected with the abdominal sac (See Nautilus, 23, April, 1910, p. 139). 
