538 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 51. 



The ^vidth of the anterior margin is one-fifth the length of the 

 lorica. The dorsal sinus is U-shaped with slightly convergent sides; 

 its depth is two-thirds of the width. Tlie ventral sinus is very large 

 and subrhomboid in outline, its widest point being some distance 

 from the front; the depth is equal to the width of the anterior mar- 

 gin. The ventral plate curves strongly downward around the 

 anterior sinus in order to give greater freedom to the movements of 

 the head. A stippled collar is present on both dorsal and ventral 

 plate. 



The foot groove varies from the wide, nearly parallel-sided outline 

 of figm^e 5 to the narrower, ovate form shown in figures 4 and 9 ; its 

 length is equal to one-fifth of the length of the body. Tlie edges of 

 the groove project below the surface of the main portion of the 

 ventral plate, at least posteriorly, as low ridges, occasionally forming 

 obtuse points on each side of the posterior emargination. 



The foot is stout and projects beyond the lorica; it is one-fifth the 

 length of the body; the terminal joint is half the length of the entire 

 foot. Tlie toes are unusually short, about one-sixth the length of 

 the lorica, tapering and slightly decurved. There is a sensory pit on 

 the dorsal side of the last foot joint. 



Total length, 190 m; length of lorica 155 m, width 120-130 /x; width 

 of anterior margin, 25-32 yu; depth of dorsal sinus, 20 fx; depth of ven- 

 tral sinus 30-35 n, width 30-32 n; length of foot groove 42-45 n, great- 

 est width 30 fx, posterior width, minimum, 18 /i; length of foot 38 n, of 

 terminal joint 18 /x; length of toes, 32 n; depth of body, 30 ^u. Figure 

 4 represents a small variety found near Atlantic City, New Jersey. 

 Its dimensions are: Total length, 148 m; length of lorica 120 fx, width 

 96 m; width of anterior margin, 24 fx; depth of dorsal sinus, 14 /z; depth 

 of ventral sinus 27 n, width 27 /x; length of foot groove 36 ix, greatest 

 width 24 n, posterior width 12 /i; length of foot, 30 fx, terminal joint, 

 15 m; length of toes, 22 fx; depth of body, 25 fx. 



Lepadella ovalis is easily recognizable by its large size and the 

 strongly depressed lorica, as well as by the relatively small opening 

 for the head and the very short toes. While this species is quite 

 variable, the variations are between the races found in different 

 localities rather than between the individuals found in any one 

 locahty, so that in order to obtain the entire range of intermediate 

 forms it is necessary to examine specimens from as many stations as 

 possible. The narrow form illustrated by figures 5 to 8 is the one 

 occurring at Washington; the nearly circular variant of figures 9 and 

 10 is from Yellowstone Park, Wyoming; the small form, figure 4, is, 

 as noted above, from Atlantic City, New Jersey. The species is 

 common in weedy ponds the world over. 



In the study of the Elirenbergian species greater dependence has 

 been placed upon his figures than on the measurements given in the 



