104 



Annals of the Carne(;ie Museum. 



very short. Thus there are seven strite between each pair of the 

 original simple stride, and theoretically, 113 striae on the adult of an 

 individual which, in the initial plicated stage, had 15. Very fre- 

 quently, however, some of the stride are suppressed, and this is almost 

 always the case at the sides. In the larger specimens, more stride are 

 probably added in this same way (Fig 23). 



In the other variety, arctistriatus, the method of development is the 

 same, but the result is somewhat different. In the earliest striated 

 stage there are from 15 to 19 sharp striae. New strire appear as before, 



but come in at earlier stages, thus 

 covering the surface of the valve 

 more comi)letely, and giving a more 

 uniform appearance to the striae 

 (Diagram, Fig. 23). 



Cardinal Ai-ea. — The height of 

 the cardinal area varies greatly with 

 the individual specimens. Series 

 can be selected in which the ventral 

 valves are subpyramidal while others 

 have so low or curved an area that 

 the valves are almost uniformly con- 

 vex. One specimen, with a hinge 

 width of I mm., has an area .36 

 mm. in height. One 5 mm. wide, 

 at the hinge, has an area of 2 mm. 

 in width. An adult, 16 mm. on 

 the hinge line, has an area 8.3 



4S6Y 



Fk;. 23. Diagram showing the 

 method of inception of plications in the 

 two varieties of Oithothetes chemungen- 

 sis. The seven lines in the middle rep- 

 resent the seven stria; as developed be- 

 tween two primary stria; in the variety 

 pecte7tacea, and the other lines represent 

 the groups of strict in the variety arcti- 

 striotus. Notice that No. 4 comes in 

 later in the middle group than in the 

 outer ones, 2 and 6 later in the middle 

 than 2 and 6 in the outer ones, etc. 



mm. high. Another with a hinge 

 width of 22 mm., has an area only 4 mm. high. 



The area is divided by two oblique lines extending from the beak 

 to the hinge. These lines, with the boundaries of the deltidium, form 

 two scalene triangles, one on each side of the delthyrium. The area 

 within these triangles is somewhat different from the rest of the car- 

 dinal area. It is striated vertically as well as horizontally, and, in 

 some specimens where the rest of the area is punctate, this part is 

 not so. The youngest specimens show no trace of these triangles but 

 a specimen 3.6 mm. in width at the hinge shows the distal ends of the 

 oblicjue lines i mm. from the extremities and the other ends close under 

 the beak. In a specimen 5 mm. in width the triangles occu])y a 



