THE NAUTILUS. 117 



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 (PI. E. Sar- 

 gent) ; Washington, D. C. (E. Lehnert). The radiating "teeth' 1 

 are of exactly the same type found in Gastrodonta lamellidens Pils. 

 a species of very different form. 



SOME STUDIES ON THE MORPHOLOGY OF THE CYCLADIDJE. 



BY DR. V. STERKI. 



1. It has been said that there are two cardinal teeth in the right 

 valves of Pisidium amnicum Miill. and P. virginicum Gniel., 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 cardinals. 



3. The whole hinge is reversed, laterals and cardinals. 



As Mr. Walker says, th.e 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 ihe 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 whoi 



*THK NAUTILI'S, IX., p. 135. (April, is- it;. 



LISRAR 



