jF'ossils of the Niagara a7id Clinton Groups, 135 



^circular, wider than high, and nearly equivalved. The surface is covered 

 ■with numerous fine elevated lines or radiations. Tliese curve outward as 

 they proceed upward, and some of them run out on the hinge line. The 

 •area is narrow, and in length but little more than one-third of the width of 

 the shell. The ventral valve is somewhat flattened near the base, and it is 

 provided with a small beak which curves slightly over the area. The beak 

 tif the dorsal valve scarcely rises above the area line. There are a few fine 

 concentric lines scarcely visible. The depression in the ventral valve is 

 •accompanied by a corresponding elevation on the dorsal valve. 



Figs. 2 and 3, [Spirifer radiatus,) Clinton and Niagara Groups. — 

 This is a fossil v/hose form is exceedingly variable, and is found in the Silu- 

 rian rocks of both England and America. We shall therefore give the 

 descriptions of authors in both countries, changing the names of the valves : 



Professor Hall thus describes it : — "Shell variable in form, sub-trian- 

 gular, rotund or subglobose, valves almost equally convex, the beak of the 

 ventral valve more or less extended, and curving over the dorsal valve, hinge 

 line often less than the width of the shell, the extremities being rounded, 

 •surface marked by fine close radiating strire, mesial elevation and depression 

 moderate, marked by the striae as in other parts of the shell, dorsal area 

 more or less exposed, and giving a very variable appearance to the shell, 

 foramen narrow and long, often partially or entirely closed by a callosity, 

 ."interior plates of ventral valve near together, and extending downwards 

 within the limit of the mesial depression." * 



Figure 2, shews a specimen with the ventral valve extended into a very 

 liigh and curved beak, with a large area beneath. In Fig. 3, the beaks of 

 the two valves are nearly of equal height, and so curved together that the 

 area is nearly closed. Some of the specimens are twice the breadth of fig. 3, 

 and with the angles more rounded. The mesial sinus, or depression, is always 

 in the ventral, or larger valve, and the mesial elevation on the dorsal, or 

 shorter valve. In fact this is their situation in nearly all the Brachiopoda. 

 The following is Professor McCoy's description of the English specimens : — 



" Transversely subrhomboidal gibbous, hinge-line slightly less than the 

 width of the shell, cardinal angles obtusely rounded, ventral valve with a 

 large incurved beak, and a wide deep rounded mesial hollow, extendiflg from 

 the apex to the front margin, which is abruptly raised into a deep quadrate 

 sinus, sides gibbous, dorsal valve with a very prominent rotundato-quadrate 

 mesial ridge, strongly defined from the beak to the sinus in the front margin, 

 sides tumid, surface radiated with vei-y fine, close, nearly equal, thread-like 

 stria?, occasionally (23 in 3 lines, at 6 lines from the beak,) casts of ventral 

 valve shew the slightly diverging slits of two exti^emely thick dental lamella?- 

 Average width, IV^ inch." 



" Common in the Ludlow rock. Keeper's Lodge, Golden Grove, Llan- 

 ileilo, Caermarthenshire." f 



This fossil on account of its extremely variable form occasions much 



* Palaeontology of New York, vol. 2, page 66. 

 t Sedgewick & McCoy's British Palaeozoic Fossils, page 195« 



