CARDIOMORPHA ORBICULARIS. 265 



groove in casts. This at first is bevelled at the expense of its lower border, the 

 upper border being prominent, but in the posterior third the plate again becomes 

 flatter. There are no signs of hinge-teeth. Above the extreme edge of the 

 valve is a narrow elongate groove, which follows the curvature of the shell, and 

 forms a narrow elongate escutcheon. The external ligament occupied the 

 anterior part of this groove. 



Exterior. — The surface is almost smooth, but under the microscope fine 

 regular lines of growth are to be distinguished. 



Dimensions. — The type specimen, PI. XXII, fig. 6, measures — 



Antero-posteriorly. Dorso-ventrally. From side to side. 



90 mm. . 80 mm. . — 



PI. XXII, fig. 5 . 68 mm. . 62 mm. . 46 mm. 



Localities. — England : the upper beds of the Carboniferous Limestone, Castle- 

 ton and Park Hill, Derbyshire ; Settle, Yorkshire ; AVorston, near Clitheroe, 

 Lancashire. Scotland : the Upper Limestone series of Newfield, High Blantyre, 

 Blatkgate. The Lower Limestone series of Gameshill, Stewarton. Ireland : the 

 Carboniferous Limestone of Tuogh, Ardlaman, co. Limerick. 



Observations. — This species was described by M'Coy in 1853, and subsequently 

 the description was republished with a figure, but the species does not seem to 

 have been widely recognised, as apparently it is not mentioned in any of the 

 lists of fossils of Great Britain. As M'Coy points out, "this is only likely 

 to be confounded with the 0. oblonga, Sow. sp., but is distinguished by its large 

 anterior and small posterior sides, extremely large beaks, and flattened orbicular 

 valves." 



I have placed four species described by de Koninck as synonyms of this 

 species. Of 0. suhquadrata it is remarked that " cette espece ressemble beau- 

 coup a Oardiomorpha orbicularis, F. M'Coy, mais elle est moms arrondie, plus 

 haute, et sa surface n'est pas aussi lisse que celle de l'espece qui vient d'etre 

 citee." Our specimens show that M'Coy's species does vary in the degree of 

 curvature of its lower border, and in the proportions of the transverse and dorso- 

 ventral diameters. 



Oardiomorpha globata is stated to have its anterior extremity larger than the 

 posterior. This is the case in the younger examples of 0. orbicularis, for with 

 growth the posterior end enlarges more rapidly than the anterior. The relation of 

 the posterior to the anterior part of the shell depends largely on the angle of view 

 at which the shell is placed. The more the posterior part of the hinge-line is 

 elevated, the smaller in consequence is the anterior extremity, and vice versa, 

 for the division between anterior and posterior is arbitrary, and is generally 

 defined by a perpendicular line falling from the umbones, and in shells with 

 arcuate hinge-lines there is no special horizontal line that can be taken as fixed. 



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