THE NAUTILUS. iba hie 
containing 1 to 3 low, radial teeth, forming transverse barriers on the 
basal wall, and appearing when the shell is viewed from the base as 
white radial stripes. Jackson county, Alabama, on hills (H. E. Sar- 
gent); Washington, D. C. (EK. Lehnert). The radiating “teeth ”’ 
are of exactly the same type found in Gastrodonta lamellidens Pils. 
—a species of very different form. 
—— + @> + 
SOME STUDIES ON THE MORPHOLOGY OF THE CYCLADIDE. 
BY DR. V. STERKI. 
1. It has been said that there are two cardinal teeth in the right 
valves of Pisidium amnicum Mull. and P. virginicum Gmel., while 
all the other Pisidia have only one, and a group has been founded 
mainly on that character. Examination of numerous specimens of 
both species have shown me that that feature is only apparent. In 
young and half-grown shells the cardinal teeth of the right valves are 
single, just as in other species, only more curved, and as they grow 
older there is a slight indentation in the middle. There the growth 
of the tooth ceases, while both ends keep on growing, until at last there 
are apparently two teeth, which, however, can usually be seen more or 
less distinctly coherent, even in mature mussels The same character 
has often been noticed in specimens of P. variabile and compressum, 
where the ‘‘two teeth” were sometimes completely separated. 
2. Reversed hinges. A few years ago Mr. Bryant Walker pub- 
lished some interesting notes** about abnormal hinges in Sphaeria. I 
had made some observations on the same subject, and have continued 
doing so since. Three different arrangements were found: 
1. The posterior laterals are reversed. 
2. The anterior laterals and the zardinals. 
3. The whole hinge is reversed, laterals and cardinals. 
As Mr. Walker says, the posterior laterals and the cardinals alone 
were never seen reversed, nor both pairs of laterals alone, nor did I 
see the anterior laterals alone, nor the cardinals alone reversed. Evi- 
dently the anterior laterals plus the cardinals form a kind of a unity, 
being situated in front of the ligament, and when one part of them are 
reversed all are so, while the posterior laterals stand alone. And the 
reversion does not only affect the numbers of the teeth, but their whoie 
*THE NAUTILUS, IX., p. 135. (April, 1896.) 
