Atrtpa eeticulaeis, Linn. 



Hespeler, T. C. Weston, 1871 : two specimens of the ordinary form 

 and one small valve with few and distant nodulous ribs, resembling the 

 variety figured by Davidson in the " Silurian Brachiopoda " (PI. xiv., 

 fig. 22,) as "approaching in character A. aspera." 



MONOMERELLA OVATA. (JST. Sp.) 

 Plate 2, fig. 1, & plate 8, flgg. 1, la, 16, & Ic. 



Shell inequivalve, the ventral valve being much larger than the dor- 

 sal : outline ovate as viewed laterally, the greatest bteadth being a 

 little in advance of the middle : valves regularly convex or with a faint 

 mesial impression on each: maximum thickness through the closed 

 valves in some specimens equal to, and in others slightly exceeding 

 their greatest breadth. Umbo of the ventral valve tumid, gibbous and 

 prominently arched, its beak being curved strongly and abruptly in- 

 wards and down to the centre of the posterior margin of the hinge plate : 

 umbo of the dorsal valve smaller than that of the ventral and not 

 nearly so prominent nor so much cui'ved. Surface marked by rather 

 coarse, ij-regular and concentric lines of growth. Test very thick pos- 

 teriorly, but gradually becoming much thinner towards the anterior 

 margin. ' 



Characters of the interior of the dorsal valve unknown. So far as 

 they can be ascertained at present, the markings on the interior of the 

 ventral valve are as follows : The outline of the hinge plate or car- 

 dinal area is crescentic or semi-circular, its posterior margin being 

 broadly and convexly rounded and its anterior border correspondingly 

 concave. The hinge itself is very broad, fiat, and closely as well as con- 

 centrically striated. In some specimens, the breadth of the hinge area 

 in the centre, and as measured from back to front, exceeds half an inch. 

 The deltidium and deltidial slopes are obscurely indicated by a faint 

 depression in the centre of the cardinal area, and by equally faint divari- 

 cating, impressed lines. 



The cardinal facet is narrower than the hinge plate : the outer mar- 

 gin of the cardinal facet is concavely and rather deeply emarginated on 

 both sides of the cardinal buttress : the exposed portion of the cardinal 

 buttress extends from the centre of the front margin of the cardinal 

 facet nearly as far as the inner margin of the anterior boundary of the 

 platform, as a narrow and acutely pointed septum, whose altitude as 

 well as breadth diminishes rapidly towards the front : the platform, 

 which is not vaulted, is feebly developed, obscurely defined and scarcely 



