40 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 9, 



ating ribs, with distant, imbricated, squamose plates. Ears small, 

 equal. 



Dimensions. — Length f-^ inch ; height |- inch. 



Affinities and Differences. — P. glauconeus is very closely allied to 

 P. subaratus, Nilsson, an Upper Chalk species of Sweden and Ger- 

 many, from which, however, it may be distinguished by its more 

 regularly rounded form, its fewer and thicker ribs, and the less 

 number of squamose asperities. From P. imbricatus, Desh., and P. 

 subimbricatus, Miinst., it is distinguished by its circular form and 

 fewer ribs. 



Locality. — The Glauconitic Sands of Woodburn and Whitehead, 

 where it is rare. Zone of Exogyra conica, Hibernian Greensand. 



12. Arca ALB^E-CREX-i:, spec. nov. 



A. radiata, Sow., in Portlock's Geol. Eep. Londonderry, &c., p. 442; 

 non A. radiata, Miiuster, 1838. 



Locality. — Eare at Lisburn ; " not rare in the Tamlaght Chalk." 

 Upper Chalk. 



13. Pholadomta obliquissima, spec. nov. PI. IV. fig. 3. 



Shell oblique, arcuate ; umboues very far forward, elevated, and 

 approximate ; anterior side very short, nearly obsolete ; posterior 

 extremity very much produced, arcuate, and subtruncated. Lunule 

 small, slightly cordate ; hinge-area large, open, and deeply impressed. 

 Surface covered by numerous st:aight longitudinal ribs, decussated 

 by well-marked lines of growth and intermediate striae. 



Dimensions. — Total length 3-| inches ; gTcatest depth 2-1- inches ; 

 height 2-1- inches ; length of anal side 3-1- inches. 



Affinities and Differences. — The above species is closely allied to 

 P. EsmarM, Kilss., from which it may be distinguished by its more 

 flattened and oblique anterior side and by its less arcuate posterior 

 extremity. P. obliqaissima is much higher proportionally to its trans- 

 verse length than P. EsmarL-i, as seven to six, and is of less depth, 

 in the proportion of three to four. 



Locality. — Not uncommon in the White Limestone (Upper Chalk), 

 Kilcorig, Lisburn, co. Antrim, and Dungiven, co. Derry. 



14. Pholadomya cordata, spec. nov. PI. IV. figs, la, lb. 



Shell oblique, subtrigonal, anterior margin keeled and curved ; 

 lunule largely cordate, produced, enclosing between it and the keeled 

 margin a deep concave area becoming shallower and more expanded 

 towards the front. Radiating ribs numerous, the anterior ones 

 coincident with the curvature of the keel, crossed by thick, concentric, 

 inequidistant lines of growth ; umbones incurved. 



Dimensions. — Total length 2^ inches ; height 2^ inches ; length of 

 anal side 2 inches, depth 1| inch. 



Affinities and Differences. — P. cordata bears a great resemblance 

 to P. decussata, Phiil. ; but the anterior extremity in that species is 

 flat, and the radiating ribs on the sides are straight in consequence, 

 and not curved as in P. cordata. The greatest diameter of P. decus- 



