126 



Among the forms most difficult to determine, are the 

 numerous smooth or finely striated Terebratuloid shells, 

 having either ovoid, elongate, sub-circular or transverse 

 forms. Among the genera of one family which in recent 

 times have been established and proposed to receive these, 

 are Athyrts (=Spirigera), Merista (=Camarium), Meris- 

 tella and Charionella ; while the subdivisions of the 

 Terebratuloid forms in another direction has given Tere- 

 bratula proper, Terebratulina, Waldheimia, Terebra- 

 tella, Centronella, Cryptonella, Renssel^eria, etc. 



The first four are of the Athyroid type, and have internal 

 spires, as in Spirifer. The shell in all these is fibrous, and we 

 have therefore in the external shell the means of separation from 

 those of the other type. 



In all the latter group we find modifications of the in- 

 ternal appendage, called in Terebratula the loop ; but in 

 none of them do spires exist. Moreover, in all these the ex- 

 ternal shell is punctate; and we do not yet know a punctate 

 shell of the external character here indicated, which con- 

 tains internal spires. 1 



The external characters, therefore, of the Terebratuloid 

 forms may be made useful in indicating the family relations 

 of the species, and may prevent us from referring to the 

 family of Spjriferid^e those which belong to the family of 

 Terebratulid^e. 



In the Thirteenth Report on the State Cabinet, published 

 in 1860, I proposed the name of Meristella for certain 

 forms which I regarded as separable from Athyris and 

 Merista ; and for the semi-plicated forms otherwise of 

 similar character, I suggested the name Leiorhynchus. At 

 the same time I described under Terebratula the follow- 

 ing species: T. lincklceni, T. reclirostra, T. lens and T. plani- 



1 The plicated forms of Retzia and Rhynchospira are of course not in- 

 cluded in the designation above made. The NucleospircB also approach the 

 terebratuloid forms, but these shells have an area on the ventral valve and a 

 different hinge structure. 



