68 
PALEONTOLOGY OF NEW YORK. 
will not justify its overthrow, as long as the type-species is a well known form 
whose structure is now thoroughly understood; and as Meristina is a term 
which has come into quite general use, it may not be cast aside for a later 
term. 
Accepting the foregoing interpretation of the characters of Meristina, we 
find but very few species which may be classed with those already discussed. 
The form described by Billings as Athyris Blancha, from the peculiar fauna of 
Square Lake, Maine (which contains, as far as known, a commingling of fossils 
elsewhere characterizing the Lower and Upper Helderberg horizons), seems to 
be a species closely allied to Meristina tumida, more so than to M. Maria. Its 
expression is unlike that of Meristella, though the character of the loop is 
undetermined. To the representatives of Meristina must be added the little 
Fig. 53. 
Fig. 52. 
The primary lamellm and loop of Meristina rectirosfra, Hall, (c.) 
species Meristella rectirostra, Hall, from the Niagara fauna, a form which has the 
biturcated loop, but is peculiar in the immature expression of its adult charac¬ 
ters, namely, very small size, high, erect beak and unclosed delthyrium. 
the other (from Frojel) is of larger size, transvei'sely subelliptical in outline, with low, broad, and slightly 
incurved beak. 
It appears from Mr. Davidson’s figures (Silurian Brachiojioda, pi. xi), that both forms occur in the 
English faunas. The characters of the American species differ from those which the Swedish and English 
specimens possess in common, in the following respects: (a) the abruptness of the cardinal slopes ; (b) the 
obsolescence or absence of a median groove over the anterior portion of the brachial valve and its lesser 
development in the sinus of the pedicle-valve; (c) the general habit of the American species is uniform and 
its expression that of neither of the variations of the Swedish species just described, but of an intei'mediate 
character. The American form thus vai-ies to such a degree that, while recognizing it as the representative 
of Atrypa tumida, it will serve a useful purpose to retain the original specific name. 
