CARDIOMORPHA EGERTONI. 275 



binge-line, contiguous, and subcentral. There is no lunule, but a narrow elongate 

 escutcheon exists, in the front part of which is lodged the external ligament. 



The shell is regularly curved, and is gradually compressed into its margins. 

 Owing to the direction of the umbones, and the depressed anterior end, the 

 general gibbosity appears to be somewhat oblique, and there is an approach 

 to compression at the postero-superior angle. 



Interior. — The anterior adductor muscle-scar is large, ovate, and very incon- 

 spicuous, remote from the margin, in the anterior umbonal hollow. The 

 posterior scars almost obsolete, situated on the dorsal slope. The hinge consists 

 of a thickened plate at right angles to the valve, which leaves a groove in casts ; 

 edentulous. Pallial line entire, almost obsolete. The surface of casts is marked 

 by obscure, fine, radiating stride. 



Exterior. — The surface is ornamented with numerous well-marked but fine 

 concentric lines of growth, with an occasional deeper one at irregular intervals. 

 Shell thick. 



Dimensions. — Pig. 4-, PI. XXV, the type of M'Coy's " Cyprina Egertoni," 

 measures — 



Antero-posteriorly . . . .97 mm. 



Dorso-ventrally . . . .87 mm. 



Elevation of valve . . . .33 mm. 



Localities. — England : the Carboniferous Limestone of Narrowdale, Stafford- 

 shire; Castleton, Derbyshire; Lowick, Northumberland; Scotland: the Carbo- 

 niferous Limestone series of Longniddry, Haddingtonshire. Ireland : the 

 Carboniferous Limestone of Millicent, co. Cork ; Nanteenan, co. Limerick ; and 

 St. Dooghlas, co. Dublin. 



Observations. — This species is easily recognised by its orbicular shape, thick 

 shell, and the small degree of inrolment of the umbones. De Koninck placed 

 this species in the genus Pachydomus of Morris, established in 1845 for some 

 large bivalves from New South Wales. I am at a loss to understand on what 

 grounds he thought it right to place a shell with an edentulous hinge-plate in a 

 genus whose hinge is described as follows by Morris in Strzelecki's ' New South 

 Wales and Van Diemen's Land,' p. 271 : — " Hinge-line sunk, with an antiquated 

 area, and one or two (?) large teeth in each valve." In this genus the muscle- 

 scars are very well marked. 



De Koninck says of the genus, " Dents nulles, remplacees par un etroit bour- 

 relet lisse." The hinge is, in fact, so very similar to that of Cardiomorpha, 

 described by the same author, that I have retained the shell in this family. If in 

 the future it should be thought that the absence of inrolled umbones and the 

 orbicular form are sufficient to separate it from this genus, it is possible that it 

 may have some affinity to Paracyclas of Hall. G. corrugata, and 0. orbicularis 



