SANGUINOLITES ANGULATUS. 371 



regularly ovate outline, less prominent oblique keel, and a smaller but deeper 

 concave dorsal slope. The concentric ribs on the valve are less regular, new ones 

 arising and others disappearing. It has occurred to me that possibly S. argutus 

 might be a young shell of S. Omalianus ; but the latter has not such an acute 

 keel and is less transverse and more gibbose. 



As far as my experience goes >S'. argutus is one of the rarest Carboniferous 

 Lamellibranchs. 



Sanguinolites ANGULATUS, de Koninck. Plate LXII, figs.] — 4. 



Sanguinolites ANGULATUS, de Koninck, 1S85. Ann. Mus. Eoy. Hist. Nat. Belg., 



vol. xi, p. 71, pi. xvi, figs. 4 and 18. 



Specific Characters. — Shell small, inequilateral, transversely ovate, strongly 

 carinate, rapidly compressed along the dorsal slope, narrower behind than at the 

 centre. The anterior end is gibbose, and its margin regularly rounded. The 

 inferior border forms the longest diameter of the shell, and is very gently convex, 

 making a well-marked angle behind with the posterior margin. The latter is 

 obliquely truncate from above downwards and backwards, and almost straight, 

 making an obtuse angle above with the hinge-line. The dorsal margin is almost 

 straight, somewhat depressed posteriorly, much shorter than the inferior margin. 

 The umbones are small, tumid, incurved, and twisted forwards; placed very far 

 forwards, and only slightly elevated. Passing downwards and backwards from 

 the umbo to the postero-inferior angle is a well-marked angular keel, separating a 

 rapidly compressed and hollowed dorsal slope from the rest of the valve, which is 

 regularly convex. Lunule small, but deep and cordate. Escutcheon large and 

 elongate, bounded by an angular curved ridge. 



Interior. — As yet unknown. Pallial line entire and remote from the margin. 



Exterior. — The surface is marked by regular, fine, concentric lines of growth, 

 more apparent near the lower margin. The region of the umbones and the dorsal 

 slope is almost smooth. Shell thin. 



Dimensions. — PI. XLII, fig. 3, from the Carboniferous Limestone of Thorpe 

 Cloud, measures — 



Antero-posteriorly . . . .17 mm. 



Dorso-ventrally . . . .8 mm. 



Thickness of valve . . . .3 mm. 



Localitij. — The Carboniferous Limestone of Thorpe Cloud, Derbyshire. 



Observations. — De Koninck has figured two specimens of this species : one, 

 a very large one, from the Carboniferous Limestone of Pauquys; the other from 



