PLATE XVIII 

 CAMAROTCECHIA WHITII Hall (Pages 344-346) 



Figure 1. — The earliest observed stage of growth, the shell measur- 

 ing 2.75 mm. in length by 2 mm. in width. The deltidial plates have 

 already begun to form along the edges of the triangular foramen, and 

 four pairs of plications are visible on the dorsal valve. 



Figure la. — Outline profile of the same. 



Figure 2. — Dorsal view of a normal adult having two plications on 

 the fold. Natural size. 



Figure 2«. — ■ Profile of the same. 



Figure 2b. — Front view of the same. (28lh llepl. N. Y. Stale Mas. 

 Nat. Hist., pi. 26, figs. 23, 26, 25.) 



Figure 4. — The cardinal features of a young example with a length 

 of 3.25 mm. ; showing the inception of the deltidial plates. 



Figure 4a. • — Profile of the same. 



Figure 5. — Cardinal features of an individual which has attained a 

 length of 6 mm. The umbo has become incurved and the development 

 of the plates is well advanced, but it is arrested at this stage, the foramen 

 not becoming enclosed at maturity. 



Figure 5a. — Outline profile of the same. 



CAMAROTCECIilA NEGLECTA Hall (Page 341-344) 



Figure 6. — Dorsal view of the youngest shell observed ; having a 

 length of .75 mm., and a width of .5 mm. Though the foramen is open 

 and without evidence of thickened margins, four fine plications have 

 already appeared on the dorsal valve. 



Figure Oa. — Outline profile of the same. 



Figure 7. — Dorsal view of an individual with a length of 2,25 mm. 

 The deltidial plates are in an incipient condition and the plications of 

 the surface have considerably increased. 



Figure 7a. — Outline profile of the same. 



Figure 8. — Dorsal view of a mature example. Natural size. 



Figure 8a. — The same in profile. (28//j Repl. X Y. State .l/«s. Nat. 

 Hist., pi. 26, figs. 3, 5.) 



Figure 3. — A somewhat abnormal adult, having three plications on 

 the fold and the lateral plications obsolescent. X 2. (11th Ann. Ucpt. 

 Geol. Surv. IniI., pi, 27, fig. 3.) 



