116 CARBONIFEROUS LAMELLIBRANCHIATA. 



situated well within the posterior margin at a point about midway between the 

 cardinal and lower borders. The pallial line is punctate, entire, and remote from 

 the margin. The hinge-plate is thickened and striated transversely. 



The Exterior. — The surface of the shell is marked with fine regular lines of 

 growth, which arise in front close together, and arching over the ridge become 

 separated and reflected upwards in subparallel curves, to terminate in the upper 

 border. The surface is almost smooth in some specimens, with regularly dis- 

 tanced subimbricating lines on the lateral surfaces. 



Localities. — England: In Millstone-grit, Pule Hill, Marsden, Yorkshire. 

 Scotland : Upper Limestone, Auchinbeg ; Middle Ironstone series, Inkerman, 

 Paisley ; Lower Limestone, Carluke ; in the Lingula Ironstone and First 

 Kingshaw Limestone ; Craigenglen ; Sculliongour, Campsie ; Upper Limestone, 

 Bankend, Lesmahagow ; Limestone, East Kilbride ; top of the Calciferous- 

 sandstone series, St. Monans, Fife. Ireland : Arenaceous Shale, Drumcurren, 

 Kesh, co. Fermanagh. 



Dimensions. — PI. IV, fig. 9, the original of M'Coy's figure, measures — 

 Greatest oblique diameter . . .58 mm. 



Length of hinge-line . . .35 mm. 



Depth of each valve . . .6 mm. 



Observations. — The type specimen of this species was obtained from Irish 

 beds, and consists of a single valve, the left, — not, as depicted in the original 

 drawing, a right valve. It is embedded in matrix, and not a perfect and free 

 specimen, as the drawing would lead the readers to suppose. The possession 

 of a small lobe anterior to the umbonal swelling is more characteristic of 

 Naiadites than Myalina. The oblique ridge, however, is anterior, and at 

 its front edge the valve is sharply deflected inwards ; and in entire specimens 

 the lobe is not so well developed. It would seem to be a form connecting 

 the two closely allied genera Naiadites and Myalina. Referred to Avicula 

 by M'Coy (to which genus it has only a very superficial resemblance), it has 

 been catalogued in this genus by Etheridge, but was referred to Myalina 

 by Armstrong and Young in their work, ' Carbonif. Fossils West of Scotland,' 

 p. 53. In many Scotch collections this name is given to M. Flemingi (M'Coy, 

 sp.). This species is not recognised as occurring in Belgian beds by de Koninck ; 

 and, though apparently rare in any one British locality, has a wide horizontal 

 distribution. There appears to be a morphological plan in the specific variations 

 of the genus Myalina, similar to that which obtains in Naiadites. There are 

 forms of Myalina which, while possessing the generic characters of Myalina, also 

 possess the general shape of N. modiolaris, N. carinata, N. triangularis, and N. 

 quadrata. Myalina Verneuilii corresponds to N. modiolaris, being more regularly 

 and transversely triangular, with a hinge-line as long as the shell or even longer. 



