68 



PALAEONTOLOGY 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 



Fio. 62. Fig. 53. 



The primary lamellsa and loop of Meristina rectirostra. Hall. IC.) 



species Meristella rectirostra, Hall, from the Niagara fauna, a form which has the 

 bifurcated 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, transversely subelliptical in outline, with low, broad, and slightly 

 incurved be.ak. 



It appeai-s from Mr. Davidson's figures (Silurian Brachiopoda, 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 ovei' 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 intermediate 

 character. The American form thus varies 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. 



