TERATOLOGICAL NOTES 67 
their edges a transverse striation commonly to be observed in 
many Uniones. 
Most of the Unionide now in the Museum of the Upper Iowa 
University were collected in the Wapsipinicon River near Inde- 
pendence, Iowa. A considerable number of the specimens, especially 
those referable to such abundant and widely distributed species 
as L. ligamentina, L. recta, and L. ventricosa, show to a greater 
or less extent the development of a third tooth, just behind the 
normal posterior pseudocardinal (pc in figures). A large series 
of specimens shows the development of this third tooth in a number 
of species. It varies in size from a simple rounded caruncle on 
the usually smooth dental bridge, to a tooth as large or even 
larger than the normal posterior pseudocardinal. At Figure 2 
is shown such a specimen, No. 483, U. I. U. Museum. The third 
tooth (apsc in Fig. 2), is very strongly developed. The connect- 
ing bridge, instead of being smooth, is raised into a sharp ridge, 
appearing a little as if it might be a rudimentary second lateral. 
The normal lateral tooth does not differ in any material respect 
from those in specimens in which the third pseudocardinal is 
not developed. 
It is impossible at the present time to say whether this ab- 
normality is caused by unfavorable embryonic conditions thus being 
truly teratologic./ I am inclined to think, however, that it is merely 
a reversion, since it occurs so commonly among the freshwater 
mussels. True teratism, such as transposition of the teeth, etc., 
occurs quite rarely, and but little is known at the present time as 
to the causes of such abnormalities. Its study offers an interest- 
ing field for qualified workers. | 
Samuel W. Getser, 
Fayette, Iowa. 
EXPLANATIONS OF THE FIGURES.* 
Fig. 1. Normal right valve of Lampsilis ligamentina (Lam.) 
Fig. 2. Right valve of Lampsilis lgamentina, showing third cardinal 
tooth. aamz, anterior adductor muscle impression; ac, anterior pseudo- 
cardinal; apsc, third pseudocardinal; arpmz, anterior retractor pedis 
muscle impression; bc, beak cavity; Jat, lateral hinge tooth; lg, 
ligament; Jun, lunule; pamz, posterior adductor muscle impression; 
pc, normal posterior pseudocardinal; //, pallial line; prpmz, posterior 
retractor pedis muscle impression; w, umbo. 
* Figures are all 14 natural diameter, 
