352 
PALAEONTOLOGY OF NEW-YORK. 
The shell when preserved shows fine concentric striae, and towards the margin 
a few crowded imbricating folds : the latter are frequently preserved in casts of 
the interior. In many casts there ai’e no more than fifteen or sixteen plications 
visible, those towards the margins being obsolete. 
In young and half-grown specimens, found associated with the others, there are 
sometimes but two plications in the sinus and three on the mesial fold (rarely a 
single one in the sinus); while sometimes there are four plications in the sinus, 
when the shell is only of medium size. 
The casts show a distinct mesial septum reaching nearly half the length of the 
dorsal valve, and in the ventral valve a short rostral cavity with short dental 
lamella. 
This species, though usually preserving its distinctive characters, presents con¬ 
siderable variety of aspect in its different conditions, and in different sediments. 
The plications are usually subangular or obtusely angular ; but in some specimens 
they are quite rounded above, while in others they are abruptly angular, and the 
entire shell has a less expanded form. It often happens, in those forms with more 
angular plications, that there are no more than five or six on each side of the 
mesial fold and sinus ; and even the broader forms sometimes present this charac¬ 
ter. The plications on the lateral portions of the shell are sometimes grooved. 
This species assumes not only the varieties of aspect represented in the illustra¬ 
tions on Plate lv, but many others not shown in the figures. The forms illustrated 
in the Report of the Fourth District, and referred-to as varieties of Atrypa lati- 
costa of Phillips, are apparently all of this species, and not identical with the 
European form. 
The figures of Phillips (loc. cit.), Plate xxxiv, 153 c, d, bear a more near 
resemblance to some of the forms of our species ; but the figures of R. laticosta , 
Phillips, as given by Mr. Davidson,* do not correspond with our species ; while 
the illustrations of R. pleurodon by the latter author, on Plate xra, figures 11, 12 
and 13, resemble the prevailing form of our species : figures 12 and 13 repre¬ 
senting those found in the poorer greenish micaceous shales of the Chemung 
group. The identifications with R. laticosta , as illustrated by Mr. Davidson, there¬ 
fore cannot stand; that species being more nearly like our R. orbicularis , which 
in turn presents many of the characters of the larger and more robust forms of 
R. contracta. It also becomes extremely difficult to point out differences between 
this species in its various phases, and some of those forms which occur abundantly 
in the Waverly sandstones of Ohio, and I am at present compelled to regard them 
as identical. 
Monograph of British Devonian Brachiopoda, Plate xiv, figures 1-3. 
