Tropidolki'ius Fauna at Canandaicua Lake, N. Y. 141 



Nepionic Shell. — In the nepionic stage, the shell is oval in outline 

 and broader than long, lioth valves are convex, the ventral a little 

 deeper. The hinge width is less than the greatest width below. The 

 surface of the shell is smooth with no fold, sinus or plications. On 

 one specimen the nepionic shell is .45 mm. long and .73 mm. wide. 

 On another it is .38 mm. long and on a third it is .60 mm. in length. 

 46 47 48 



Fu;. 46. Spirifer mucronatus Conrad ; outline of protegulum and shell in nepionic 

 stage ; taken from a dorsal valve. X 28. 



Fig. 47. Spirifer mu^ronatiis Conrad. A ventral valve showing the shell in the 

 stage in which there are only two plications beside the ridges bounding the sinus. X 3- 



Fig. 48. The same species. Ventral valve at the stage when there are six plica- 

 tions beside those bounding the sinus. >' 2. 



Neanic Stages. — In the earliest neanic stages a sinus is developed 

 in the ventral valve, bounded by two strong ridges which are the 

 first plications. Following this, a fold is formed in the dorsal valve 

 and plications are added on the margins in pairs, each pair coming in 

 outside the older ones. The later plications added do not reach to 

 the beaks. 



The smallest shell in the series, a ventral valve 1.06 mm. long and 

 1.48 mm. wide, has the median sinus, the two plications bounding it, 

 and two very short lateral plications. The delthyrium is wide and 

 without deltidial plates. 



The ventral valve, in early neanic stages, is about twice as deep as 

 the dorsal. 



When the shell is about 2 mm. in length an impressed line appears on 

 the fold of the dorsal valve and, soon after, a faint fold or plication 

 appears in the ventral sinus and grows constantly stronger throughout 

 life. 



In an early neanic stage the hinge width becomes the greatest 

 width of the shell and the cardinal extremities soon become acumi- 

 nate. Specimens up to five or six mm. in width do not have the 

 strong mucronations but nearly all adolescent shells above that size 

 have them. In one series measured the index increased constantly 

 from 1.30 in a shell 1.73 mm. in length to 3.5 in an adult 14.8 mm. 

 in length. This last shell did not have the mucronate cardinal ex- 

 tremities. 



