LITHODOMUS LINGUALIS. 75 



expands upwards and downwards, as it passes backwards. Below the swelling 

 in the middle third of the valve there is a broad shalloAv constriction corre- 

 sponding with the concavity in the inferior border. The hinge-line of each valve 

 is depressed anteriorly, so that there is a concave surface between the umbones 

 of the opposing valves posteriorly ; this widens out to become lost on the 

 posterior slope of the shell. In this way the edge of the valve commences 

 below the umbo and is directed downwards, but by a slow twist on itself it 

 gradually rises and becomes directed u|)Avards posteriorly, where it comes 

 into close apposition Avith the corresponding joortion of the opposite valve. 



The left valve appears to have been more convex than the right. The shell is 

 very thin. 



The internal character of the hinge is partially shown in a specimen in the York 

 Museum (PI. II, fig. 2). It appears to be edentulous, and the ligament to be marginal. 

 The anterior adductor scar is small, excavated, marginal, situated just below and in 

 front of the umbones. The j)osterior adductor scar is almost obsolete, and repre- 

 sented by a broad flattened area equidistant from the posterior superior and inferior 

 borders. 



The Exterior. — The shell is ornamented with very fine, close-set lines of growth, 

 which are crowded in front, and, following the contour of the margins of the shell, 

 terminate in the hinge-line. As the lines separate in passing backwards, other fine 

 ones appear in between, and towards the ventral margin there are occasionally 

 deeper wrinkles dividing the stria? into bundles ; but these are not continued far 

 posteriorly. 



Dimensions. — The tyjDe specimen of Etheridge's Modiola lithodomoides {L. 

 lingualis), PI. I, fig. 4, measures — 



Antero-posteriorly . . -. .90 mm. 



Dorso-ventrally at termination of hinge-line . 29 mm. 



Lateral elevation of valve . . .11 mm. 



Localities.— l^nghmd : Carboniferous Limestone of Park Hill, near Longnor, 

 Narrowdale, Castleton, and Wensleydale. Ireland : St. Dooghlas, co. Dublin ; 

 Millicent, co. Cork ; and Clonmel. Scotland : Limestone, East Kilbride. 



Observations. — I have arrived at the conclusion that many shells described under 

 various names by paleontologists really belong to one and the same species, and 

 conform readily to the figures and description given by Phillips for his Modiola 

 lingualis. The original is unfortunately lost, but there is a very well-preserved 

 specimen labelled M. lingualis in the Gilbertson Collection noAv in the British 

 Museum (Natural History), which may be taken as fairly typical, for it was 

 from the fossils collected by Mr. Gilbertson that Phillips drew many of his 

 types. The original specimen is said to have come from Castleton, where T have 

 obtained others, which thus forge another link in the chain of identification. The 



