278 BRITISH PALAXOZOIC FOSSILS. [LAMBLLIBRANCHIATA. 
These fossils seem almost perfectly identical in form and thinness of the shell with Myacites (Plewromye) 
of the Mesozoic rocks, but are distinguished by the inflexion of the dorsal margins to form a posterior lunette 
as in Pholadomya, and differ both from the former and latter in the entire, or unsinuate, pallial impression 
and the two dorsal furrows. Their real affinity is with the genus Sangwinolites, of which probably they 
merely form a subgenus, only distinguished by the short rhomboidal form, although this certainly gives them a 
very distinct aspect from the typical elongate forms; in addition to which the absence of the thick posterior 
cardinal ridge of that genus is to be noticed. 
LEPTODOMUS AMYGDALINUS (Sow. Sp.) 
Syn. and Ref. = Cypricardia ? amygdalina Sow. Sil. Syst. t. 5. f. 2. + C. 2retusa id. t. 5. f. 5. 
Sp. Ch.—Obovate, shell very thin, width varying from three-fifths to half of the length (in the latter case 
modified by pressure); very tumid towards the anterior end, where the depth of both valves about equals the 
width; beaks very large, tumid, close to the anterior end, which is subtruncate more or less obliquely, with 
a deep cordate lunette below the beaks; posterior end rounded or subtruncate, varying in obliquity with the 
length; ventral margin slightly convex, and with a faint concavity in the anterior third; hinge-line not 
elevated, the inflected portion of the edge narrow, concave, nearly at right angles to the plane of the valves ; 
surface with fine lines of growth. Length of average size and shape eleven lines, proportional width *, 
greatest depth of both valves (at one-third below the beaks) /%. 
This seems to vary greatly in the proportion of length to width and depth, owing to the extreme thin- 
ness of the shell offering little resistance to pressure; one specimen from near Ludlow exactly agrees in all 
respects with Sowerby’s above-quoted figure, but it is obviously distorted by pressure, giving a greater length, 
narrowness and depth to the valves, a sharp diagonal ridge, and more pointed posterior end; but other 
obviously identical specimens in the same mass not being so squeezed, seem much broader, shorter, and with 
a slight impression, affecting the anterior third of the ventral edges, and having the general proportions which 
I have given above, which were taken from a nearly unaltered specimen from Brigsteer, and which seems a 
fair average example. As a general rule the more compressed (or laterally flattened) the specimens are, the 
broader and shorter do they seem; and I have satisfied myself that the change from mere physical pressure 
of the form given by Sowerby as @. amygdalina (exaggerated in length and depth by vertical pressure, or 
squeezing of the dorsal and ventral margins towards each other) into his @. retusa (in which the width is 
greater and the depth less than natural, from the opposite or lateral pressure) is to be traced with ease 
and certainty. The young is proportionally shorter and subtrigonal, nearly approaching the form of the 
C. retusa (Sow.), but without the depression behind the anterior end, and not so flattened. 
Position and Locality—Common in the Upper Ludlow greenish mudstone near Ludlow, Shropshire, and 
in the hard micaceous quartzites of the same age, at Benson Knot, Kendal, Westmoreland, and Brigsteer, 
Kendal, Westmoreland ; in the dark schists of Cwm Craig Ddu, Builth, Brecknockshire. 
LEPTopOMUS GLOBULOSUS (J/°Coy). Pl. 1. L. fig. 11. 
Ref—Id. M°Coy, Ann. Nat. Hist, 2nd Series, Vol. VII. p. 57. 
Sp. Ch.—Globose, subtrigonal, width three-fourths of the length; beaks very large, a little nearer the 
anterior than posterior end; sides evenly tumid, most so in the middle; posterior slope undefined, but very 
steeply sloped; anterior and posterior ends subequal, slightly contracted, rounded, ventral margin convex ; 
hinge-line a little shorter than the shell, not elevated, inflected portion narrow; surface with a few concentric 
lines of growth. Length seven and half lines, proportional width *,, depth of one valve “. 
This departs so widely from either the shortest or most gibbous varieties of the Z. amygdalina (Sow. sp.), 
that it seems desirable to give it a distinctive name ; there is no other closely allied form. The general appear- 
ance approaches that of the Nucula ovalis of the same rock, but in the latter the diagonal posterior ridge is 
more angular, and I have ascertained that it really possesses teeth as in Vucula. 
