PACKAED.] GEOLOGICAL SUCCESSION OF PHYLLOPODA. 357 



measures of Britain and tbe 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 

 Molluscan shells, but possibly Orustaceau and Phyllopodous. 



"I have some specimens from the upper Coal-measures of Ard wick, 

 near Manchester (collected by Professor Williamson, F. R. S., 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), L. 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 

 bebind, 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 aeross 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 middle area, and vertically 

 straight or slightly curved, and wider apart, on the posterior j)art 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 Cypricardise and other shells 

 of Palaeozoic age to which this little fossil has some resemblance in shape ; 

 and some Urthonotse have a general resemblance to it; but some of the 

 small Astartes of the Chalk and Oolite, small as the A. Eoemeri, Miiller's 

 Petref. Aachen, Kreideform, Plate 6, fig. 12, and A. interliiieaia, Morris 

 and Lycett, Mollusca of the great Oolite (Palfeontog. 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 (of 

 Goniatites ?) ; but there is little or nothing to support the hypothesis. 



