MYALINA FLEMING! 113 



diameter of the shell. The umbones are narrow, acute, carinated, and terminal, 

 contiguous and deflected forwards. The lateral surfaces of the shell are suddenly 

 cut off in front, where they meet the anterior surface at an obtuse angle, and this 

 is the most tumid portion of the shell. The carina forming the angle between the 

 anterior and lateral surfaces is subacute in section, convex in profile, and is directed 

 downwards and backwards to meet the anterior edge of the lower border, becoming 

 near the lower edge of the shell less angular and somewhat expanded. The lateral 

 surfaces are compressed from their anterior border rapidly into the hinge-line above, 

 but more gradually in the other directions. The greatest lateral measurement is 

 about the centre of the keel. 



Interior. — The apex of a cast does not correspond with the external point of 

 the shell, and is not terminal, and is separated from the anterior extremity by a 

 sulcus occupied by the rostral plate. The posterior adductor scar is moderately 

 large, remote from the posterior border, situated about the junction of the upper 

 and lower halves of the shell. The mantle-line is entire, remote from the border. 

 The hinge-plate is thickened and transversely striate. 



Exterior. — The surface of the shell is ornamented with fine laminae and lines of 

 growth, which are crowded and directed obliquely downwards on the anterior surface 

 till they reach the borders, when they are rapidly deflected and curved upwards 

 over the lateral surface to terminate in the hinge-line and upper portion of the 

 posterior border. 



Dimensions. — The type specimen, a single valve, fig. 9, PI. IV, measures — 



Obliquely antero-posteriorly. Length of binge-line. 



38 mm. 19 mm. 



Another specimen, fig. 10, PI. IY, measures — 



Obliquely antero-posteriorly. Length of binge-line. From side to side. 



31 mm. 13 mm. 14 mm. 



Localities. — England : From a bed of Millstone-grit, Pule Hill, Marsden, 

 Yorkshire. Ireland : The limestone of Millicent, co. Cork ; Clane, co. Kildare. 

 Scotland : G-arngad Road section, Glasgow. 



Observations. — The type of this shell is preserved in the Griffith Collection, 

 Museum of Science and Art, Dublin, and a figure of this specimen is given at 

 PL IV, fig. 9. The original drawing does not give a very accurate idea of the 

 characteristic shape of the species, owing to the fact that a view has been given with 

 the specimen placed obliquely, so that the angle between the anterior and lateral 

 surfaces is not apparent. The drawing has not been reversed on the stone, so 

 that the right valve instead of the left appears. M'Coy's original description is 

 " Trigonal, length twice the width; hinge half the length of the shell, depressed, 

 smooth ; anterior face truncated, bounded by a slightly sigmoidal ridge ; beaks 



15 



