302 MEMOIRS OF THE CARNEGIE MUSEUM 
stands rather isolated, but the gaping margins are found at least in one other 
species, Plagiola securis (Lea) (See Pl. LX X XIX, fig. 17).8 Here the general shape 
is similar to the third type, but it is rather distinctly spatulate and quite large. 
I think that the glochidium of Plagiola securis is the only one which might be 
compared with that of Lampsilis alata, and it is rather significant that the only 
glochidium, which permits such a comparison, is found in a group, which, as we 
shall see later, is rather closely allied to Proptera. 
The third type of glochidium is found in all the rest of the genera of Simpson 
investigated by myself. Here the shell is semicircular or semielliptical, the ventral 
margin evenly rounded, and no spines are present. With the exception of Plagiola 
securis the margins shut completely. There are minor differences in size and shape; 
as has been mentioned, those of Paraptera gracilis are very small (See Pl. LXX XIX, 
fig. 19). Of those of the genera Quadrula, Pleurobema, Unio, Ptychobranchus, and 
Obovaria, we may say, that they are generally rather small, while those of Lampsilis 
and Truncilla are somewhat larger. 
The following is a list of the Pennsylvanian species, in which I have seen 
glochidia of this type: Truncilla triquetra Rafinesque (Pl. LX X XIX, fig. 24); Trun- 
cilla perplexa rangiana (Lea); Lampsilis ventricosa (Barnes) (Pl. LXX XIX, fig. 23); 
L. ovata (Say); L. cariosa (Say); L. multiradiata (Lea); L. luteola (Lamarck); L. liga- 
mentina (Lamarck) (Pl. LXX XIX, fig. 16); Z. orbiculata (Hildreth) (Pl. LXXXIX, 
fig. 22); L. recta (Lamarck) (Pl. LXXXIX, fig. 21); L. nasuta (Say); L. iris (Lea) 
(Pl. LXXXTX, fig. 20); L. gracilis (Barnes) (Pl. LXX XIX, fig. 19); Obovaria cir- 
culus (Lea) (Pl. LX XIXX, fig. 15); Plagiola securis (Lea) (Pl. LXX XIX, fig. 17); 
Ptychobranchus phaseolus (Hildreth) (Pl. LXXXIX, fig. 14); Unio crassidens La- 
marck (Pl. LXXXIX, fig. 6); Unio gibbosus Barnes (Pl. LXX XIX, fig. 7); Pleuro- 
bema clava (Lamarck) (Pl. LXXXIX, fig. 5); Quadrula undulata (Barnes) (PI. 
LXXXIX, fig. 3); Q. coccinea (Conrad) (Pl. LXX XIX, fig. 4); Q. rubiginosa (Lea) 
(Pl. LXXXIX, fig. 2); Q. subrotunda (Lea); Q. kirtlandiana (Lea) (Pl. LXXXIX, 
fig. 1). 
Other Pennsylvanian species, the glochidia of which are known to be of this 
type (See Lea, Observ. VII and X), but have not been seen by myself, are: Lamp- 
silis radiata (Gmelin), L. parva (Barnes), and Obovaria retusa (Lamarck). 
3. The breeding season. 
Lea (Observ., II, II, VII, X) had already paid attention to the time of 
breeding of the Unionide. Sterki (1898 and 1903) has pointed out that there are 
8See footnote on page 300. 
