82 CARBONIFEROUS LA^IELLIBRANCHIATA. 



the shortest transverse diameter of the valve. The umbones are obtuse, flattened, 

 the beaks lying a little in front of the centre of the hinge-line. 



The valves are gently convex, separated behind from the dorsal slope by an 

 angular fold, so rapid is compression of the narrow triangular dorsal slope. 



Interior unknown. 



Exterior. — The surface is covered by fine regular concentric strias and lines of 

 growth, crossed in the anterior two-thirds by obsolete radiating striae, and thus 

 has a crenulate appearance. In the posterior third the radiating lines become 

 stronger and stronger as they pass backwards, so that they form several well- 

 marked raised angular ribs on that part of the valve just anterior to the 

 posterior slope. 



Dimensions. — PI. VI, fig. 8, from Glencartholm, measures — 



Antero-posteriorly . . . .16 mm. 



Dorso-ventrally . . . .15 mm. 



From side to side . . . .3 mm. 



Locality. — Scotland : the Calciferous Sandstone series of Glencartholm, Esk- 

 dale, Dumfries. 



Observations. — This is a very small shell which is not at all uncommon in the 

 scorpion beds of Glencartholm, Eskdale. The shell, when not thoroughly exposed 

 at the hinge, has the appearance of a cardiform shell, but the square hinge and 

 small ears demonstrate its real affinities. I have referred the shell to 

 Posidonomya on account of the oblique character and angulated dorsal slope, but 

 in the absence of details of the interior it is impossible to be actually certain. Z'. 

 radiata is not likely to be mistaken for any other Carboniferous bivalve, but it may 

 be mentioned here that the species has no connection with P. cos^ato, M'Coy. I am 

 very doubtful if this shell deserves the name " costata," for the radiating ribs are 

 most probably quite accidental, and due to foldings and pleats in the thick perios- 

 tracum. These folds are irregular in number and distribution, and examination 

 with a lens shows that M'Coy was not correct in stating that there were " four 

 longitudinal, distant, well-defined mesial folds." The shell probably belongs 

 to Posidoniella minor. Brown, 1841, of which this name must be considered a 

 synonym. It was obtained at Rush, co. Dublin, and I refigure the type of M'Coy's 

 species now preserved in the Griffith Collection, Museum of Science and Art, 

 Dublin, PL VI, fig. 10. 



