LAMELLIBRANCHIATA. | UPPER PALAXOZOIC MOLLUSCA. 495 
way to Professor Bronn’s prior name. The extremely minute projections in the hinge of the left valve bounding 
externally the pits for the teeth of the right valve, seem too insignificant to be counted as teeth, even in the 
most strongly marked cases, and generally they are quite inappreciable. In two of the carboniferous species 
(M. axiniformis Phill. sp. and M. carbonaria Sow. sp.) Professor King has found the thick tooth of the left 
valve undivided. 
MYOPHORIA CARBONARIA (Som. Sp.) 
Ref. and Syn. = Venus carbonaria Sow. Geol. Trans. 2nd Series, Vol. V. t. 39. f. 2;?+ Donaa sulcatus id. id. f. 1. 
(not Venus (Amphidesma) carbonaria Portk. Geol. Rep.) 
Desc.—Var. a. Rotundato-rhombic ; beaks very large, obtuse, prominent, subcentral; anterior side very 
broadly rounded ; posterior end short, subtruncate, with slight obliquity, narrowed by the convergence of the 
hinge-line and ventral margin; ventral margin slightly convex in the anterior half, gently concave in the pos- 
terior half in front of the diagonal ridge; valves very gibbous near the beaks, gradually arching from thence to 
the ventral margin, and rapidly arching to the anterior margin; a wide, shallow concavity extends from near 
the beak to the shallow sinus in the posterior part of the ventral margin ; posterior slope abruptly flattened, 
leaving an obtusely rounded diagonal ridge. Surface with a few obscure concentric plicee and lines of growth ; 
a slender sulcus usually bisecting the posterior slope nearly along the middle. Length one inch three lines, 
50 
width the same, proportional width of posterior end about ;;;, length of hinge-line about “4, length of anterior 
end about =, depth of one valve =. 
Var. 8. Ovato-oblong ; beaks very large, tumid, oblique towards the anterior end; anterior end very short, 
rounded ; posterior end elongate, gradually compressed ; posterior margin obliquely subtruncate, slightly convex ; 
posterior slope abruptly flattened, leaving an obtuse diagonal ridge from the beak to the respiratory angle 
(bisected nearly along the middle by a small sulcus) ; ventral margin convex in the anterior half, nearly straight 
in the posterior half; valves very tumid at half the width from the beaks, and at one-third the length from the 
anterior end, from whence the valves arch rapidly towards the beak, the ventral and the anterior margins, 
gradually flattening towards the posterior end (no distinct sinus in the margin, nor hollow in front of the 
diagonal ridge). Length one inch three lines, in proportion to length greatest width from beak to opposite 
ventral margin ;;;, width of posterior end about ;, length of hinge-line about =, length of anterior side about =, 
depth of one valve 3. I 
The first of these varieties is even shorter in proportion than Sowerby’s figure of his Venus carbonaria, and 
with a rather more strongly marked sinus in the posterior ventral margin, and corresponding hollow in front of 
the diagonal ridge; while the second variety is even more elongate and inequilateral than his figure of his Donazx 
sulcatus, and has the same elongate posterior end, straight posterior ventral margin, and all the other characters, 
yet I am absolutely certain that the two extremes pass insensibly into each other, and are specifically identical. 
Professor King found the original specimens to belong to his genus Schizodus (= Myophoria), and both have 
the sulcus from which the Donaz sulcatus received its name. All our specimens seem covered with a thick, 
brownish, periostraca. Muscular impressions as in the generic character. 
Position and Locality —Common in a carboniferous limestone resembling that of Burdiehouse, at the coal- 
pits S. of Berwick-on-Tweed. 
MYOPHORIA DEPRESSA (Porth. Sp.) 
Ref. and Syn. = Amphidesma or Mactra depressa Portk. Geol. Rep. t. 36. f. 8. (beak too near the 
posterior end). 
Dese—Rotundato-rhombic ; beaks moderate, prominent, a little nearer the anterior than the posterior end ; 
anterior end rounded, compressed; ventral margin gently convex in the anterior part, with a very faint con- 
striction in front of the respiratory angle, which is obtusely rounded ; posterior end obliquely subtruncate, gently 
[easc. 11. ] 
