132 



E. A. ANDREWS. 



along the sternum and, apparently, of no use as yet. The annulus, 



however, has acquired a rather ma- 

 ture appearance. When enlarged 

 fifty diameters, Fig. 19, it shows 

 I i toward the center two gentle eleva- 



\\( ^* tions to represent the future tuberosi- 

 " ties and posteriorly a transverse curved 

 rim like that of the adult. The whole 

 plate is still narrow from before back 

 but much less so than in previous 

 stages. The suture of the receptacle 

 is indicated by the zig-zag line and its 

 lateral walls by the broken lines. The 



former represents : anteriorly on the left the former edge of the 

 hood ; posteriorly the closed up groove ; and in its middle course 

 the transverse fold. 



Enlarged 200 diameters, Figs. 20 and 21, the receptacle 

 presents varieties of development in different individuals. The 

 groove itself may be open posteriorly as in Fig. 20 or quite closed 



FIG. 18. 



FIG. 19. 



up as in Fig. 2 1 . The anterior part of the groove bends far to 

 one side to end under the hood at the right end of the transverse 

 fold. This part of the groove may be still accessible by passing 

 in under the transverse fold, Fig. 20, or it may be quite shut off 

 by a posterior fold growing along against the posterior edge of 

 the transverse fold as in Fig. 21. Fig. 20 is not much advanced 

 beyond the preceding stage, Figs. I 5 and 16, the open groove, the 

 hood and the transverse fold are larger and the invaginated shell 

 is more extensive, as indicated by the broken lines. But in other 

 cases, Fig. 21, the deeply buried groove connects with the sur- 

 face only by oblique planes that come to the surface as the three 



