Figure 11. — The cardinal area represented in figure 5 (length 

 1.25 mm.), still further enlarged ; showing the broad, prominent, exsert 

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



FiGURB 12. — The cardinal area shown in figure 6 (length 2 mm.), 

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

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

 the sheath and the callosity. 



Figure 13. — The pedicle-area of a mature individual. The sheath 

 is now wholly absorbed, the sole trace of it being seen in the csecal 

 foramen, surrounded by the umbonal portion of the shell. The callosity 

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

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

 the dorsal groove. X 2. 



ORTHOTHETES SUBPLANUS Conrad (Pages 327-330) 

 See Plate XXII 



Figure 14. — Ventral view of the smallest individual observed ; 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 14a. — Outline profile of the same ; showing the convexity of 

 the valves. 



Figure 15. — A normal adult ; dorsal view. 



Figure 1.5a. — The same in profile, {op. cit., pi. 21, figs. 30, 31.) 



Figure 16. — 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 16. 



Figure 19. — 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 ]iiargius of 

 the ventral aperture. 



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

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

 is very large, nearly filling the aperture between the valves, and the del- 

 tidial plates have attained the maximum development observed in the 

 Strophomenida3. 



The last thi'ee figures have the same degree of enlargement. 



