144 Annals of the Carnf.gie Museum. 



From Thedford, Ont., there are in the collections of the Peabody 

 Museum a great number of well-preserved specimens belonging to 

 this species. They can be separated into three well-marked varieties, 

 but there is a great amount of variation within the limits of these 

 varieties. 



First and least common is the form very much like the specimens 

 from Canandaigua Lake. It is short but wide. The front is regu- 

 larly curved and the cardinal extremities, while acute, are not extended 

 into spiniform projections. The dorsal fold has a depressed median 

 line, but there is no corresponding plication in the ventral sinus. The 

 index is from 2.16 to 3.00 ; length, 6 to 17 mm.; width. 14 to 41 

 mm.; plications, 18 to 40. This is probably the shell given the 

 varietal name arkonsis by Shimer and Grabau,' but none of the speci- 

 mens in the Yale collection show the plication in the sinus of the 

 ventral valve. 



In the neanic stages, as shown by the growth lines, this variety is 

 short and rather strongly mucronate. 



A second variety is the long form, which is straight in front and 

 with the sides straight or slightly curved. The cardinal angles are 

 acute and the extremities somewhat prolonged in well preserved 

 specimens. This type is fairly common. The index is low, 1.60 

 to 2.00. The length from 15 to 20 mm., the width from 25 to 35 

 mm., and there are iS to 30 plications, usually about 22 or 24 on 

 each valve. 



The third and most common form is nearly semicircular in outline, 

 with the cardinal extremities somewhat produced. The shell has a 

 tendency to become somewhat robust, especially in the later stages. 

 The index varies from 1.85 to 2.92. The more common range is 

 from 2.05 to 2.30. There are usually from 22 to 30 plications, though 

 there may be as few as 14 on small individuals and as many as 40 

 on large ones. In the adult, some have the depressed line in the 

 fold of the dorsal valve and some do not. This is the variety thed- 

 fordensis of Shimer and Grabau <yIoc. cif.). 



The index here given is too low, as the cardinal extremities are so 

 worn and broken in the adult that it is difficult to say exactly what 

 the mature form was. But, judging from the growth lines, it is prob- 

 able that the extremities were acuminate, not, however, nearly so ex- 



' " The Hamilton group of Thedford, Ont., Shinier and Grabau." Bull. Geol 

 Soc. Am., Vol. 13, pp. 149-186, June, 1902. 



