NOTICE OF PALEONTOLOGY OF NEW YORK. 197 



I have continued the Terehratula lens under that genus, without knowing 

 its internal structure, but from its external similarity with those which 

 have proved to belong to Ihat genus. 



These investigations have clearly shown how difficult, or even impos- 

 sible it is to refer to their proper generic relations these fossils from 

 external form and character alone. 



The following species are retained under this genus : Cryiotonella recti- 

 rostra, C.])lanirostra^ C. ipMs (n. s.), C. ImcMceni* and a doubtful form, C. 

 (^Terehratula) eudora^ from the Chemung group. 



Under the Genus Centronella are described the C. glans-fagea, C. 

 alveata, C. impressa and C. glaucia (n. s.). 



A farther study of the genus Tropidoleptds has revealed certain 

 features of internal structure, which seem to ally it with the Terebratu- 

 lidae. 



The typical species of this genus is a concavo-convex shell, having the 

 general form of Lept^na and Strophomena , and was originally described 

 by Mr. Conrad as Stroplmnena carinata. It differs from all the genera of 

 Strophomenidai in both external and internal characters, and, for these 

 reasons, has been separated. The shell is externally strongly ribbed, 

 and the tisxture is finely punctate throughout its substance. The ventral 

 area is well defined, narrow and linear. The fissure or foramen is very 

 large and wide, and is excavated above the area line, coming quite up to 

 the beak, and sometimes even including the apex which is worn away or 

 absorbed. 



The teeth, which are a little separated from the margins of the fora- 

 men and not continuations from it, are strong and thickened below, while 

 they are deeply crenulated on the summit and exterior margins. There 

 is a narrow low median ridge in the cavity of the valve ; and the div^ari- 

 cator muscular impressions have not been satisfactorily observed. 



The dorsal valve has a narrow area, and a wide and strong cardinal 

 process which nearly or quite fills the foramen of the opposite valve. 

 This process is often simple exteriorly, above the limit of the smooth or 

 striated pseudo-deltidium which covers it near the hinge-line ; but just 

 within the valve it is broadly grooved in the middle, usually with two 

 small deep pits just within the external smooth callosity, and on each 

 side there is a groove and accessory lobe, frequently not conspicuous. 

 The divisions made by the median groove diverge and terminate below 



* This species has proved to be a true Terebratula. 



