996 PALEONTOLOGY OF NEW-YORK. 



The plications on this shell are usually simple, though in a few specimens one 

 or two of them are seen to bifurcate. The surface of perfect specimens would pro- 

 bably show fine concentric lines; but those hitherto found, have such markings 

 only near the margins of the valves. As is usual in this type of Rhynchonella, the 

 plications in front and at the sides are marked with a central impressed line to- 

 wards the margins of the valves. 



In form and general aspect this species varies greatly, being sometimes longitu- 

 dinally ovate or oblong, in others globose and subpentagonal. The extremes of these 

 varieties, without the intermediate forms, would appear to present well-marked 

 specific differences; but a careful stujy of the series shows such an imperceptible 

 gradation of form as to leave no doubt of their identity. 



The cast of the ventral valve shows a strongly marked ovate muscular imprint, 

 the details of which, and of the accessory parts, vary in individuals of diflerent 

 forms and proportions. 



PLATE XXIX. 

 Fig. 4 a - c. Young shells having a more elongate form than the prevailing types. 

 Fig. i d - k, and p. The shorter form of this shell in the smaller individuals. 

 Fig. 4 1-0. The ovoid forms of this species. 



PLATE XXX. 



Fig. 1 a - e. Ovoid forms of this species. 



Fig. 2 a, b, c. Profile views of the more gibbous forms, showing the gradual expansion of 

 the dorsal valve. 



Fig. 2 d, e. Front and dorsal views of a very gibbous form. 



Fig. 2f - q. Illustrations of extremely gibbous forms, which sometimes present a sub- 

 quadrangular outline, and some variation in the form of the broad extension of 

 the mesial sinus in front, and in the proportional length and breadth of the 

 dorsal valve as shown in figures 2 / and 2 o. The form 2 w is rare, but is ne- 

 vertheless one which the species sometimes assumes. 



The preceding figures of Plate xxix and Plate xxx represent a series of forms, 

 which can be traced from the smaller to the larger individuals through uninterrupted 

 gradations. This variation is not confined to this species alone, but I have found 

 other species of the genus, when examined in extensive collections, to be represented 

 by a more elongate and a more gibbous and rotund form. 



Fig. 2 a, 6 and c represent the changing direction of the line of junction in the 

 two valves, and the increasing ventrlcosity of the dorsal valve; while the ventral 

 valve retains more nearly its original form, and is sometimes even proportionally 

 less gibbous than in smaller forms. The larger specimens are nearly all from the 

 Pentamerus limestone, and have the shell partially exfoliated, so that the finer 

 details of surface marking are not preserved. 



Geological position and locality. In the lower pentamerus limestone of the Lower 

 Helderberg group : Helderberg mountains, Schoharie, Carlisle and other places. 



