TERATOLOGICAL NOTES 65 
CHASEA FLEXILIS (Gatt.) Nwd. 
Panicum flexile (Gatt.) Serib. 1893. 
Panicum capullare var. flexile Gattinger, 1887. 
Other plants belonging to the group are P. patens L; P. latifolium 
Linn., P. brevifolium Linn., and many more.] 
TERATOLOGICAL NOTES. 
IV.—Frequent Occurrence of a Third Pseudocardinal in the Right Valve 
of Certain Species of Lampsilvs. 
During the past four years, I have examined the hinge areas 
of a large number of Umionide, and have been much impressed 
with the fact that the dentition of these freshwater mollusks 
is very variable, not only within the family and within the genus, 
but even within the species. That considerable variation exists 
is to be expected, since it is generally conceded that the teeth 
of Pelecypoda, being largely moulded by the dynamics of situation, 
change with the influences to which their form is due, and in 
the course of time may become obsolete from disuse, as in Ano- 
donta*, or may simulate those of other groups. As a rule, however, 
the type of dentition is pretty constant for the various families 
and genera, so that they may be given a definite dental formula. 
The freshwater mussels are schizodont in their dentition, their 
hinge teeth, when present, consisting of a subumbonal and a 
lateral series, known as pseudocardinals and pseudolaterals. The 
ligament is external, parivincular and opisthodetic. Umnonide 
are distributed in the lakes, ponds, and streams throughout the 
entire world, and in the Mississippian region of America we find 
their metropolis. 
In his Synopsis, the only comprehensive recent work dealing 
with the great family of Naiades, Simpsonf{ defines the dentition 
of the genus Lampsilis in the following characterization: ‘‘ Hinge 
with one or two pseudocardinals and one lateral in the right valve, 
and two pseudocardinals and two laterals in the left.’’ Other 
* Dall, W. H. zn von Zittel, Karl A. Textbook of Palaeontology, 
Eastman’s Translation I (2): 353-354. (1900). 
+ Simpson, Charles Torrey. Synopsis of the Naiades, or Pearly Fresh- 
Water Mussels, Proc. U. S. Nat. Museum XXII: 526. (1900). 
