510 BRITISH PALAZOZOIC FOSSILS. ( LAMELLIBRANCHIATA. 
CARDIOMORPHA OBLONGA (Sow. Sp.) 
Ref. and Syn. = Isocardia oblonga Sow. Min. Con. t. 491. f. 2. 
Dese.—Longitudinally oblong, obliquely gibbous from the beaks towards the respiratory angle, from which 
line the valves arch gradually to the anterior and ventral margins, flatter and more abruptly sloped towards 
the posterior and dorsal margins; beaks moderately large, inflated, very oblique, approximate, inrolled over the 
anterior end, having a semi-cordate depression beneath them in each valve; anterior end scarcely projecting 
in front of the beaks, very small, subquadrate; ventral margin nearly straight; posterior end long, dilated, 
obtusely subtruncate, with the anal and respiratory angles rounded ; hinge-margin slightly elevated, nearly as 
long as the shell, inflected at right angles to form an external cartilage support, of nearly two lines wide, ex- 
tending nearly from the beak almost to the cardinal angle. Length two inches six lines, proportional width 
from a little behind the beaks to ventral margin 5, width of posterior end ;;, length of anterior end ,j,, depth 
of both valves =, (greatest about one third from the beaks). 
The thickness of the shell of this species in specimens of the above size is about one fifth of a line, The 
species is easily distinguished by the extremely small size of the anterior end, and the elongate, oblong, dilated 
posterior end. 
Position and Locality.—Not uncommon in the dark lower carboniferous limestone of Isle of Man. 
CARDIOMORPHA ORBICULARIS (A/‘Coy). Pl. 3. I. fig. 41. 
Ref —M°Coy, Ann. Nat. Hist. 2nd Series, Vol. XII. 
Desc.—Suborbicular ; anterior end moderately large, semicircular, compressed; ventral and posterior 
margins very convex, regularly curved; posterior side small, rounded, gradually compressed; beaks very large, 
tumid, obliquely inrolled towards the anterior end, projecting greatly beyond the hinge, nearly over which is 
the deepest part of the shell; valves becoming gradually flattened towards the margin; surface smooth, even; 
substance of the shell very thin; hinge-margin inflected at right angles, forming a cartilage support rather less 
than two lines wide. Length three inches two lines, proportional width from beak to ventral margin >, length 
of anterior end ;;,, width half way between the beak and posterior end =, depth of one valve (greatest near the 
beak) 5. 
This is only likely to be confounded with the C. oblonga (Sow. sp.), but is distinguished by its large anterior 
and small posterior sides, extremely large beaks, and flattened orbicular valves. The young of the C. corrugata 
(M°Coy), which is nearly like in form, is distinguished by the large corrugation of the sides. 
Position and Locality.—Seems not uncommon in the earboniferous limestone of Derbyshire. 
Explanation of Figures.—PI. 3. 1. fig. 41, left valve, natural size ; 41 a, ditto, seen from behind, to shew 
the inflected dorsal margins. 
Family. ARCIDZ. See page 282. 
Genus. ARCA (Linn.) See page 282. 
Subgenus. MACRODON (Lycett.) 
Gen. Char.—Shell oblong, very inequilateral, moderately tumid, a byssal sinus in the anterior third of the 
ventral margin. Anterior edges of the adductor impressions prominent (as in Cucullea); hinge-teeth at the 
anterior end few, slightly oblique, or nearly at right angles to the hinge-line beneath the beak, becoming more 
oblique towards the anterior end; all the posterior part of the hinge-line, from the beak to the anal angle, 
occupied by from one to three long lateral teeth nearly parallel with the hinge 
The following species of this genus has been referred to Byssoarca of Swainson, by Prof. King; in that sub- 
genus, however, the entire hinge-line, from one end to the other, is crossed nearly at right angles by very 
numerous, nearly uniform, subequal teeth; thus differing entirely from the dentition of the fossil in question, 
and the present subgenus, in which the posterior three-fourths of the hinge-line shew but two or three ex- 
tremely long teeth at right angles to the position of the very numerous ones occupying the same place in 
Byssoared. 
