CYPRICARDELLA. 345 



ridge bounding the muscle-scar an oblique hollow, behind which is an oblique 

 depression. 



Exterior. — The shell is covered with concentric lines of growth, which become 

 very much accentuated as they approach the oblique ridge ; on the posterior slope 

 they become flattened and markedly imbricate. The shell is of moderate 

 thickness. 



Dimensions. — Fig. 12, PI. XXXVIII, in my own collection from the upper bed 

 of Thorpe Cloud, measures — 



Antero-posteriorly . . . .73 mm. 



Dorso-ventrally . . . .45 mm. 



Elevation of valve . ' . . .15 mm. 



Locality. — England : the Carboniferous Limestone of Thorpe Cloud, Derby- 

 shire. 



Observations. — Mytilomorpha angulata attains to a fairly large size, but it is 

 much less transverse and more nearly quadrate than M. rhombea ; moreover in the 

 latter the segment of the valve in front and below the oblique keel is almost equal 

 to the upper and posterior segment, but with regard to M. angulata the latter is 

 much larger than the former, and in addition the oblique ridge is also much more 

 acute, and the anterior end is much smaller in every way. 



This species has a considerable superficial resemblance to a mytiloid shell, 

 especially to Myalina Fleming i, but the internal characters and the hinge-line show 

 that it has no real affinity to this genus. I cannot find any Belgian specimens of 

 this species, nor at present have I obtained it from any other locality than that of 

 Thorpe Cloud at the entrance to Dovedale. Here it is not at all uncommon, but 

 perfect examples are rare, and I have not yet seen a specimen possessing both 

 valves in situ. Casts of the hinge are not common, but show a simple linear 

 hinge edge posteriorly, and in front a thickened hinge-plate with indications of 

 teeth. 



Family CRASSITELLLCLE. 

 Genus Ctpricaudella, Hall, 1858. 



Venus, Phillips, 1836. Geo]. Yorks., pt. 2, p. 209. 



Miceodon, Conrad, 1842. Jouru. Acad. Nat. Sci. Phil., vol. viii, p. 247, 



Cypricardia, de Koninch, 1843. Desc. Anim. Foss., p. 97. 



Astarte (pars), de Koninch, 1843. Ibid., p. 80. 



Venus, Morris, 1843. Cat. Brit. Toys., 1st edit., p. 104. 



— Brown, 1849. Illust. Foss. Couch., p. 205. 

 Cypuicakdia (pars), d'Orhigny, 1851. Prodrome de Paleontol., p. 130. 

 Megalodon (pars), tfOrligny, 1851. Ibid., p. 130. 



