MODIOLA JENKINSONI. 71 



think it is very probable that the shell described from the same bed by this author 

 as Mytilus fabalis is a slightly smaller specimen of the same species, the descrijDtion 

 and figures resemble the former very closely. De Koninck only adopts the latter 

 species, which he says is founded on a single valve, a right one ; but de Ryckholt 

 figures a left valve, and also gives a view from above of a bivalve example. De 

 Koninck figures a right valve with a very rounded posterior border, which is stated to 

 be " I'echantillon-type decrit par ce paleontologiste " (de Ryckholt). The description 

 of M. ligonula coming before that of M. fabalis must therefore be adopted, if I am 

 correct in my surmise that the two species are identical. 



MoDioLA JenkixXsoni, J/'C'oy, 1853. PI. VIII, figs. 1, 1 a, 1 b, 2, 3. 



LiTHODOMTTS Jenkinsoni, M'Col/, 1853. xlnn. Mag. Nat. Hist., ser. 2, vol. xii, 



p. 189. 



— — M.'Coy, 1855. Classification Brit. Palseoz. Eoeks, p. 



493, pi. iii, F., fig. 2, 2a. 



— — R. Ellieridge, 1885. British Fossils, vol. i, Palaeozoic, 



p. 284.. 



— — loung and Armstrong, 1871. Carboniferous Fossils, 



Trans. Geol. Soc. Glasgow, vol. vii, 

 Suppl., p. 52. 



Specific Characters. — Shell transversely oblong, somewhat obliquely gibbose, 

 expanded and somewhat compressed posteriorly. The anterior end is very short, 

 extending very slightly in front of the beaks, and it is very narrow from above 

 downwards. Its border is obtusely rounded, passing with gradual curve into the 

 inferior border, which descends slightly and is convex doAvn wards in its anterior 

 and posterior thirds, and straight in the middle third. The posterior border is 

 obtusely and regularly rounded. The upper dorsal margin is nearly straight, 

 gradually rounding into the posterior end. The umbones are small, tumid, much 

 incurved, and twisted forwards, encroached upon in front by a wide, deep, cordate 

 lunule, and are situated in the anterior one-fifth of the shell. The valves are evenly 

 swollen, most so in front and along an undefined line passing from the umbones 

 downwards and backwards, the tumidity becoming gradually less to a point 

 anterior to the junction of the inferior and posterior borders. Shell convex, but 

 somewhat compressed along the inferior border and at the posterior superior angle. 



Interior. — In casts the hinge-line appears to be simple. The anterior adductor 

 muscle-scar is shallow and round, situated well within the margin of the shell in the 

 deep hollow beloAv the lunule, and extending inwards. The posterior adductor scar 

 is also at some distance from the posterior margin and is large, oval, and shallow, and 



