60 PALEONTOLOGY OF NEW YORK. 



Observations. This name is proposed as a substitute for the term Meristina 

 in its current application to species not congeneric with M. Maria. The num- 

 ber of these species is, probably, comparatively large, and their features subject 

 to considerable variation, though, with few exceptions, there are none having 

 the structure of the brachidium as described above, which present diflfer- 

 ences in other respects sufficient to justify a separation from the type form. 

 Heretofore the structure of the loop in this group has not been correctly dem- 

 onstrated. Mr. Davidson figured and described preparations of Atrypa nitida 

 and Terebratula didyma, Dalman* (which he regarded as equivalent terms), 

 showing a loop erect and slightly inclined backward at its tip, but without the 

 simple posterior prolongation ; he applies to these species the generic term 

 Meristina. The reasons are given elsewhere for restricting the genus Meristina 

 to species similar to M. Maria, Hall ; and though the second species mentioned 

 in the original description of that genus, Atrypa nitida, agrees rather more 

 closely in the form of the loop with the figure given at that time, both 

 species vary from the structure as there represented, which is a condition 

 not yet known to occur among the brachiopods. It is not unlikely, how- 

 ever, that this phase of development may be found among some early athyroid 

 species. 



We may with reasonable security refer to this genus the following American 

 species : Atrypa cylindrica, Hall, A. intermedia, Hall, A. naviformis, Hall, of the 

 Clinton group; A. nitida, Hall,f A. crassirostra. Hall (^ A. cylindrica. Hall), of 

 the Niagara group, and Charionella? Hyale, Billings, of the Guelph limestone. 

 With these are probably to be associated Atrypa oblata. Hall, of the Medina 

 sandstone, and Athyris Harpalyce, Billings, of the Lower-upper Helderberg 



* Silurian Brachiopoda, Supplement, pi. iv, figs. 20-23a. 



t In the original description of this species the appellation nitida was applied to a small form, elongate- 

 subtriangular in outline and subtruncate on the anterior margin. At the same time a larger foi'm with a 

 more gi'adual anterior slope was designated as var. oblata. It is the latter which agrees more closely with 

 the very abundant shell in the Niagai-a fauna of Waldron, Indiana, subsequently identified as Meristella 

 nitida (Twenty-eighth Annual Report of the New York State Museum, p. ItiO. 1879), while the typical 

 form of the species is found in the extension of the Niagara fauna to the southward, in the vicinity of 

 Louisville, Kentucky. The similarity of the Waldron variation to the Meristina didyma, as identified by 

 Davidson from the English Sihu-ian, is very close, while the typical Atrypa nitida seems to maintain per- 

 manent differences. The Gotland forms of Atrypa didyma have a higher umbo than any of the American 

 shells, constantly exposing the deltidial plates and the entire length of the pedicle-opening. 



