NOTICE OF PALAEONTOLOGY OF NEW-YORK, VOLUME IV. 165 



The species of this genus are more elongate than Merista and Meri- 

 STELLA, and those now known are less distinctly marked by mesial fold 

 and sinus ; while the beak is more attenuate, often a little flattened, and 

 rarely so closely incurved, as in the genera cited. The punctate structure 

 of the shell is a distinguishing character. 



This genus was first described as above cited, and figures of the exterior 

 form and of the interior of the valves were given in the Fifteenth Report. 

 The results of some farther investigations were given in the Sixteenth 

 Report, and an illustration of what was supposed to be the internal appen- 

 dages of the shell. About the same time it was discovered that some punc- 

 tate shells of the same general form possessed the loop of Terebratula 

 proper; and it became a matter of great interest, and still greater difiiculty, 

 to determine the internal structure of the species. I had referred to this 

 genus the Terebratula lens^ T. lincklcBni^ T. rectirostra and T. plaiiirostra^ 

 species described in a preceding report. Having ascertained that the inter- 

 nal loop in Centronella glans-fagea is essentially similar to that of C. julia 

 (Winchell), a species having the external form of Cryptonella, I 

 supposed it possible that the two genera might be merged into one. Since 

 that time, I am not aware that any farther investigations have been made, 

 tending to throw light upon this subject. 



It has therefore appeared to me very desirable to learn, if possible, the 

 internal structure of any one of those species which I originally referred 

 to the Genus Cryptonella. Of two of these, C. rectirostra and C. plani- 

 rostra, I have fortunately been able to obtain an exposition of the form 

 and structure of the loop, as will be shown in the illustrations of the 

 species. 



The crura have the general form of those in Waldhetmia, extending 

 in a long recurved loop, with long processes descending into the ventral 

 valve, between which and the apex of the shell the crura are united by a 

 transverse band, difi"ering in this respect from that genus, while the muscu- 

 lar impression and extended beak off'er other distinctions. 



I have thought it desirable, therefore, to continue the name Crypto- 

 nella to include these two authentic species ; while the other two species 

 are included under it from their general form. 



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

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

 have proved to belong to that genus. 



These investigations have clearly shown how difficult, or even impossible, 

 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 : Cryptonella recti- 

 rostra, C. planirostra, C. iphis (n. s.), C. ? lincklceni, and a doubtful form, 

 Cryptonella {Terebratula). 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 Tropidoleptus has revealed certain fea- 

 tures of internal structure, which seem to ally it with the Terebratulidae. 



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 Strophomena carinata. It differs from all the genera of 

 Strophomenidae in both external and internal characters, and, for tliese 



