3S9 PALAEONTOLOGY OF NEW-YORK. 



The shell when preserved shows fine concentric striae, and towards the margin 

 a few crowded imbricating folds : the latter arc frequently preserved in casts of 

 the interior. In many casts there are 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 

 lamellae. 



This species, though usually preserving its distinctive characters, presents con- 

 siderable variety of aspect in its different conditions, and in diflereut 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 Atryjya lati- 

 cosla of Philijps, are apparently all of this species, and not identical with the 

 European form. 



The figures of Phillips (loc. cit.), Plate xxxrv, 153 c, d, bear a more neiar 

 resemblance to some of the forms of our species ; but the figures of R. laticosta, 

 PiuLLiPS, as given by Mr. Davidson,* do not correspond with our species ; while 

 the illustrations oi R. pleurodon by the latter author, on Plate xm, 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. contractu. It also becomes extremely difiicult to point out difierences 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. 



