122 CARBONIFEROUS LAMELLIBRANCHIATA. 



an obtuse angle with the posterior border. The umbones are acute, terminal, 

 anteriorly curved, with a slightly prominent oblique swelling or ridge extending 

 from each down the anterior side and close to its margin. 



Interior. — The arrangement of the muscle-scars and the details of the hinge 

 have not yet been exposed. 



Exterior. — The surface is ornamented with fine concentric lines of growth, 

 interrupted at intervals by shallow sulci. 



Dimensions. — One of Mr. Etheridge's type specimens measures dorso-ventrally 

 16 mm., antero-posteriorly 11 mm. A flattened specimen from Abden (PI. IV, 

 fig. 15) measures dorso-ventrally 18 mm., antero-posteriorly 15 mm. 



Localities. — In hardened cyprid shale at a quarry under Craiglockhart Hill, on 

 the north side of Colinton Road, Edinburgh, Cement-stone group ; in shale of 

 the Calciferous Sandstone series at Abden; in the Yoredale Rocks of Swaledale. 



Observations. — A shell typical of the genus, but smaller and flatter than most. 

 I have in a very large measure adhered to Mr. Etheridge's very accurate and 

 careful original description. In his observations the species is compared with 

 Mytilus \Myalina~] Flemingi, Myalina lamellosa, and two American species. It 

 is then stated that " from M. Flemingi it may be distinguished by possessing 

 a smaller and more truncated anterior end, the bounding ridge from the beaks 

 being less posterior in position, a less depressed and shorter hinge-line, and a 

 more broadly rounded ventral margin." Mr. Etheridge, jun., must have mistaken 

 some other shell for M'Coy's type ; as the anterior end being obsolete, and the 

 bounding ridge from the beaks forming the margin of the flattened anterior face, 

 it cannot possibly be said to be posterior in position to that in M. sublamellosa. 

 Really the species under description is a much flatter shell, less acutely angular 

 in transverse contour, and without such a markedly expanded anterior surface 

 as that of M. Flemingi. I am enabled to figure some specimens obtained by Mr. 

 Bennie for the Geological Survey of Scotland, from the locality whence the 

 originals for Etheridge's description came, by the kindness of the Director-General ; 

 also some on a small slab, containing several examples, collected by Miss J. Donald 

 from Swaledale. 



It has occurred to me that this species may only be a dwarfed form of M. 

 Vemeuilii (M'Coy), but I have no evidence to support this assumption. The 

 name " sublamellosa " suggests an affinity to de Koninck's shell, M. lamellosa, to 

 which the species under description has no resemblance beyond a generic one. 

 The original type examples are, I believe, preserved in the Museum of Science 

 and Art at Edinburgh. 



