PLATE XV 



CRANIA SILURIANA Hall (Page 317) 



Figure 1. — The youngest individual observed ; having a height of 

 1 mm. and a widtli across the base of 1.5 mm. The elevation of the 

 shell is in strong contrast to that of the mature form. 



Figure 2. — A mature individual attaclied to a shell of Platijstoma. 

 (2Slh Rept. N. Y. Slate Mus. Nat. Hist., pi. 21, fig. 5.) 



DALMANELLA ELEGANTULA Dalman (Pages ;317-321) 

 ■ See Plate XX 1 1 



Figure 3. — Dorsal view of the youngest embryo observed, its length 

 being .5 mm., its width, .75 mm. The median sinus has ah-eady formed, 

 and three pairs of plications have appeared, of which the middle pair is 

 evidently the oldest. 



Figure 3rt. —Outline profile of the same; showing the slightly greater 

 convexity of the ventral valve. 



Figure 4. — A larger example viewed from the dorsal side, its length 

 being 1 mm., its width 1.5 mm. The plications have now increased to 

 the number of six pairs, one of which has appeared between the median 

 plications seen in figure 3. 



Figure 5. — Ventral view of an individual in about the same stage of 

 development ; showing a strong median plication corresponding to the 

 dorsal sinus, and five pairs of lateral plications. 



Figure 6. — Cardinal view of an individual in the growth-stage repre- 

 sented by figure 5. The valves have nearly the same convexity, while 

 the width of the cardinal area and the size of the pedicle-passage are the 

 same for each. The latter is seen to be quite unobstructed and without 

 further differentiation than a slight thickening of the margins. 



Figure 6rt. — Outline profile of the same. 



Figure 7. — Cardinal view of an individual which has reached a size 

 of 3 X 3 mm. Here a change is apparent in the development of the car- 

 dinal area and foi-amen of the valves. The primary indication of the 

 callosity or cardinal process is in the apex of the dorsal opening. The 

 difference in the convexity of the valves has also noticeably increased. 



Figure 7a. — Outline profile of the same. 



Figure 8. — Cardinal view at a size of 5 X 5 mm. The ventral beak 

 has become strongly incurved, and the cardinal process is now sub-divided 



