BRACHIOPODA. 77 



It may be argued, and, indeed, has been assumed by some writers, that the 

 proper type of this genus is that specified in tlie first use of the term Meris- 

 TELLA, Atrypa iiaviformis, of the Clinton group. No definition, however, of the 

 genus was given in that connection, and though we are still in ignorance of 

 the precise character of the loop in A. naviformis, it almost certainly differs 

 from that of M. lavis, and all species belonging to Meristella as this term is 

 currently applied. To adopt this species as the type would be to ignore the 

 elaborate diagnosis of the genus afterwards given as founded on M. l&vis, 

 and to cause an altogether indefensible confusion of nomenclature by giving 

 Meristella an uncertain value and requiring a new name for the extensive 

 group of species now most properly referred to the genus. 



To the genus Meristella, then, may be referred such species as differ from 

 Merista in having no shoe-lifter process, but, in its place, a very deep muscular 

 impression. In both genera, the brachial supports, which were first demon- 

 strated for Meristella, by Mr. R. P. Whitfield (Palaeontology of New York, 

 vol. iv), and subsequently for Merista, by the Rev. Norman Glass, afford no 

 satisfactory basis of distinction, although there is a slight difference in them, 



Fig. 65. no. 56. 



The loop of Meristella Walcotti. (o.) 



as indicated above. Tlie drawings of the loop here given are the first to rep- 

 resent with precision the character of the curvature of the circular branches. 

 There is also, probably, a considerable and, perhaps, a significant difference in 

 the structure of the hinge-plate of the two genera. This plate in Merista has 

 been described from the small American species, Merista Tenmsseensis, sp. nov., 

 in defiiult of any evidence of its character in M. hercuka, and from this form 



