Figure 11. — Tlie cardinal area rcprpsoiited in fii^urc 5 (length 

 1.25 nun.), still furtluT enlarged ; showing the broad, prominent, exsert 

 sheath, embracing, at its base, the faint, grooved dorsal callosity. 



Figure 12. — The cai'dinal area shown in figure G (length 2 mm.), 

 enlarged to the size of figure 11; showing tlie depression of the sheath, 

 the narrowing of the cardinal area, and the increasing aperture between 

 the sheath and the callosity. 



FiGURK 13. — The pedicle-area of a mature individual. The slieath 

 is now- wholly absorbed, the sole trace of it being seen in the ca-cal 

 foramen, suiTounded by the umbonal jiortion of the shell. The callosity 

 is strongly developed, but not sutRcieiitly to close the gap between it and 

 the opposite valve, tlnis leaving a passage between the valves and along 

 the dorsal groove. X 2. 



ORTIIOTHETES SUBPLANUS Cunkad (Pagks ;)27-;J30) 

 See Plate XXII 



Figure 1-1. — Ventral view of the smallest individual ol)served; hav- 

 ing a length of 2.25 mm. Both primary and secondary plications and 

 concentric growth-lines have already appeared; indicating the very early 

 assumption of these characters. 



Figure Ha. — Outline profile of the same ; showing the convexity of 

 the valves. 



Figure 15. — A normal adult ; dorsal view. 



Figure 15a. — The same in prohlc. (op. cil., pi. 21, ligs. '■){), :il.) 



Figure 10. — Cardinal view of specimen somewhat larger than that 

 represented in figure 14. The ventral valve bears a small pedicle-sheath, 

 the dorsal, the inception of a cardinal process or callosity, while between 

 the two is a broad opening which serves to indicate that at this early 

 age the pedicle-sheath had ceased its function. 



Figure 17. — Cardinal view of an individual slightly below normal 

 full growth, but with essentially mature characters. 



Figure 18. — The pedicle-area of the specimen represented in fig- 

 ure IG. 



Figure 1!). — Pedicle-area of a shell having a length of 4 mm. At 

 this stage of growth the sheath has relatively diminished in size, while 

 the dorsal callosity has increased and shows a median groove on its inner 

 edge. Deltidial plates have also begun to develop along the margins of 

 the ventral aperture. 



Figure 20. — Pedicle-area of the specimen represented in figuie 17. 

 The sheath is now atrophied and altogether obsolete, the dorsal callosity 

 is very larg(% nearly filling the apertin-e between the valves, and the del- 

 tidial plates have attained the maximum development observed in tlie 

 Stroi)homenidie. 



Tlie last thi'ee figures have the same decrree of enlargement. 



