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 the first use of the term Mems- 
TELLA, Atrypa naviformis, 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. IcRvis, 
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. 55. 
The loop of Meristella WalcoUi. 
as indicated above. The 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 Tennesseensis, sp. nov., 
in default of any evidence of its character in M. herculea, and from this form 
