658 



F. C. GREENE 



9 



lO 



II 



12 



13 

 14 

 15 

 16 



17 

 18 

 19 

 20 

 21 

 22 



23 

 24 



25 

 26 



27 



0.9 mmXi 

 i.o mmXi 

 i.o mmXi 



1.1 mmXi 



1.2 mmXi 



1.3 mmXi 

 1.5 mmXi 

 1.5 mmXi 

 1.5 mmXi 

 1.9 mmXi 

 1.9 mmXi 

 2.0 mmXi 

 2.25 mmXi . 

 2.25 mmXi . 

 2 . 5 mm X I . 

 2.75 mmXi. 



2.8 mmXi. 

 3.0 mmX2. 

 3.25 mmX2. 



3.5 mmX2. 

 4,0 mmX2. 



3.9 mmX2. 

 3.9 mmX2. 

 4.0 mmX2. 

 4.25 mmX2. 



4.6 mmX2. 

 4.75 mmX3- 



mm 

 mm 

 mm 

 mm 



.2 mm 

 .25 mm 

 .4 mm 

 .2 mm 

 .4 mm 

 .5 mm 

 .5 mm 

 .5 mm 

 .6 mm 

 •75 mm 

 5 mm 

 ,75 mm 

 ,0 mm 

 25 mm 

 25 mm 



4 mm 



5 mm 

 75 mm 

 5 mm 

 9 mm 

 9 mm 

 25 mm 



Ventral valve {Plate I). — The mesial sinus in the youngest specimen 

 is a deep, narrow groove near the beak. This occurs regularly up to 

 the i6th specimen, that is, one about 1.6 mmXi.2 mm in size, after 

 which it occurs only at irregular intervals. From the youngest up, it 

 moves forward so that in the older specimens it is seen on the anterior 

 margin. The striae appear distinctly for the first time on the fifteenth 

 specimen (size 1.6 mmXi.i mm). On the youngest specimens there 

 are two cardinal spines (Plate IV, Figs. 1-3, 6-8) one on each side 

 of the beak. These are located on the extremity of the hinge-line 

 in the youngest specimen, but as the hinge-hne becomes longer, the 

 spines are added successively at the extremities. No. 15 in this series 

 shows the first traces of an additional spine, the new spine appearing, 

 of course, between the original spine and the lateral margin. The 



