PACKARD.] GEOLOGICAL SUCCESSION OF PHYLLOPODA. 357 
measures of Britain and the lower Carboniferous red sandstone of 
Pennsylvania. I know nothing of their nature, except that they are 
small, thin, horny, brown, stiffly quadrate, symmetrical bodies, unlike 
Mollusean shells, but possibly Crustacean and Phyllopodous. 
““T have some specimens from the upper Coal-measures of Ardwick, 
near Manchester (collected by Professor Williamson, F. R.8., several 
years since), and some from the lower coal-measures of Fifeshire, col- 
lected by Mr. Salter, F. G.S., of the geological survey. Dr. Isaac Lea 
described and figured, a few years ago, a similar fossil from the red 
sandstone of Pennsylvania, and named it Cypricardia leidyi. All these 
three are very much alike; but, on account of the obscurity of their 
relationship, and the distant places, geological and topographical, of 
their occurrence, and making the most of their slight differences of 
contour, I propose to keep them nominally distinct as Leaia leidyi 
(Plate 5, figs. 11, 12), D. leidyi, var. williamsoniana (Plate 1, figs. 19, 
20), and L. leidyi var. salteriana (Plate 1, fig. 21) Dr. I. Lea, of Phila- 
delphia, being the first to notice and figure a specimen of this pro- 
posed genus, I have distinguished it by a name commemorative of that 
well-known conchologist. The carapace-valves are oblong; truncate 
behind, with a slight curvature of outline; boldly rounded in front; 
either straight or somewhat curved on the ventral border; straight on 
the dorsal edge; a slight umbo takes the place of the antero-dorsal 
angle, from whence two conspicuous ridges (hollow within) pass along 
the surface of the valve; one directly across the valve to the antero- 
ventral angle; the other, and longer one, passes diagonally to the 
postero-ventral angle; these ridges divide the convexity of the valves 
into three, unequal, triangular, smooth, sloping areas; the anterior space 
is the smallest, and nearly semicircular; the middle one has its apex at 
the umbo, and its base along the ventral margin; and the posterior 
space is based on the hinder margin, and reaches along the dorsal 
region to the umbo. The surface of the valve is marked with 10-13 (?) 
delicate ridges (hollow within), concentric, beginning at the umbo, con- 
formable to the outline of the valve, and sharply bent at the divergent 
ridges; they are curved and closely set on the anterior area; more open, 
horizontal, and straight, or nearly so, on the middie area, and vertically 
straight or slightly curved, and wider apart, on the posterior part of 
the valve. These symmetrical markings of concentric angular lines 
and transverse divergent ridges give this fossil, at first sight, a striking 
likeness to some fish-scales, when the two valves lie open, in contact by 
their dorsal edges (as in Plate 1, fig. 19), and produce a bilaterally 
symmetrical, subquadrate, concentrically lined figure, with triangular 
sloping areas. Dr. Lea points out some Cypricardiz and other shells 
of Paleozoic age to which this little fossil has some resemblance in shape; 
and some Crthonotz have a general resemblance to it; but some of the 
small Astartes of the Chalk and Oolite, small as the A. Roemeri, Miiller’s 
Petref. Aachen, Kreideform, Plate 6, fig. 12, and A. interlineata, Morris 
and Lycett, Mollusca of the great Oolite (Paleontog. Soc. Monograph), 
Plate 9, figs. 14, 15, have even a greater resemblance in size and shape, 
without being at all allied to the form before us. 
The horny tissue of Leaia—its long dorsal edge destitute of hinge— 
its stiff and simple style of ornament—and its two diagonal, raised hol- 
low ridges or folds, remove it from the Mollusca. It has been suggested 
(by Phillips and Williamson) that these fossils may be Trigonellites (ot! 
Goniatites?); but there is little or nothing to support the hypothesis. 
