57 



Durham, J. Townsend, 1874-82 : two pedicle valves with the test pre- 

 served, and a well preserved cast of the interior of the same valve. Speci- 

 mens which Professors Hall and Clarke believe to belong to this variety 

 have since been found in the Niagara limestone at Hawthorne, Illinois. 



MONOMEEELLA DuRHAMENSIS. (N. Sp.) 

 Plate 9, fig. 1, and pi. 15, fig. 1. 



Shell strongly convex, especially in the umbonal region, elongate subo- 

 vate in marginal outline, much longer than broad, broadest in advance of 

 the midlength, and narrow and pointed posteriorly : beaks of both valves 

 extremely large and elevated, apparently equal or nearly equal in size, 

 each with a correspondingly high cardinal area or pedicle surface. 



Pedide valve. Cardinal area obliquely flattened, slightly divergent from 

 that of the brachial-valve, subdivided longitudinally into a central and 

 four lateral portions, corresponding to the " deltidium," " deltidial ridges,'' 

 and " areal borders," of authors, by four slightly divergent linear grooves. 

 " Deltidium " fully twice as high as broad, flat, its surface marked by 

 crowded and nearly transverse raised lines, which curve slightly forward 

 and are crossed and made somewhat tuberculate by a few small and 

 feebly developed longitudinal ridges. " Deltidial ridges " rather broad, 

 nearly half as broad as the deltidium, slightly convex and transversely 

 striated, " Areal borders " narrow. Characters of the interior of this 

 valve unknown. 



Brachial valve. Umbonal region enormously thickened, the test of the 

 beak being almost solid, though composed of numerous thin laminae. 

 Cardinal area flat, not subdivided longitudinally by four grooves, as that 

 of the pedicle valve is, but striated across, and bounded externally on each 

 side by a low narrow ridge with rounded summit, like the " areal border " 

 of a Trinierella. " Cardinal facet " apparently rather narrow, though its 

 limits posteriorly are not clearly defined, curved a little forward in the 

 centre and slightly backward on each side. Immediately in front of the 

 cardinal facet, and on each side of its forward curve, there is a shallowly 

 subcrescentic excavation or depression of the hinge plate. Crown of the 

 crescent apparently narrow, its sides and ends not clearly defined. In- 

 terior of the valve deeply concave posteriorly, simple and devoid of 

 umbonal chambers. Platform sharply V-shaped ; platform vaults appa- 

 rently shallow ; anterior septum short, extending from the apex of the 

 platform to a short distance from the front margin of the valve. 



Surface of the exterior of both valves apparently smooth. 



The only specimen known to the writer is not suflficiently perfect to 

 admit of exact measurements, but, when unbroken, it was probably about 

 three inches and a half in length, and an inch less in its greatest breadth. 



