"FROM IHE INFERIOR OOLITE. 535 



This handsome species seems very like Ph. nodosa of the Obtusus- 

 zone, Goldf. Petr. derm. 1. 156. fig. 5, but its umbones are more pro- 

 minent, so that the dorsal profile of the shell presents two sweeping 

 concave curves. Mosch has sunk Goldfuss's name as a synonym of 

 Ph. idea, D'Orb., v. Deshayesii (Chap. & Dew.), though Goldfuss cer- 

 tainly has the priority. At all events, if his identification is correct, 

 our species is certainly distinct, and, indeed, belongs to a different 

 section of the genus, as D'Orbigny's shell has well-defined shield- 

 borders, which are wanting in the present one. 



Prom Sowerby's longer figure of his Ph. Murehisoni it differs by 

 not having such massive ribs and some other particulars. Mosch 

 reunites to that shell the form Ph. Heraulti, which Agassiz, followed 

 by Lycett &c, had distinguished from it. The observation of many 

 Dundry specimens has always led me to regard the latter authors 

 as correct in this ; but, in any case, it is clearly different from the 

 present species, and in fact occurs in a higher bed from that in 

 which this is found. 



Ph. jproducta, Sow. Min. Conch. 1. 197. fig. 1, approaches it more 

 closely, but in it the umbo is less central, and the rays larger, more 

 distant, and spreading over the anterior side. Mosch, loc. cit. p. 39, 

 identifies this shell with Ph. ddtoidea; and if he is right, it has not 

 the least to do with the one under consideration, being much longer 

 and altogether of another shape. 



Prom Ph. frickensis, Mosch, loc. cit. 1. 10. fig. 1, it is distinguished 

 by its much less terminal umbo and more distant rays. Compared with 

 Ph. Wittlingeri, Waag. Ben. Geogn.-Pal. Beitr. p. 614, of which there 

 is a specimen among the Inferior Oolite fossils of the British Museum, 

 and which has the same prominent second rib, it is wider, with 

 much longer and more defined umbo, and its anterior side is more 

 produced above and obliquely truncated below. However, the two 

 forms are so similar that I do not feel positive that they are dis- 

 tinct species. 



Ph. media, Ag. loc. cit. p. 72, is very like it in ornamentation, but 

 more rounded in shape, with a smaller less- defined beak and 

 obtusely bounded escutcheon. Again, it comes much nearer to 

 Ph. nymphacea, Ag. loc. cit. p. 71, in the former particulars ; but in 

 that the escutcheon is described as very large, deep, and obtusely 

 bounded, and the muscle-mark is smaller. 



Myacites subsidens, n. sp. Plate XVIII. figs. 24, 24a. 



Shell transverse, convex, but centrally flattened. Umbones rather 

 anterior, facing anteriorly, and much incurved, truncated in front 

 by a curving rounded angle, expanded backward. The cardinal 

 areas defined by prominent obtuse ridges in both directions. The 

 anterior and posterior margins deeply and nearly equally convex, 

 the inferior margin slightly so. The larger portion of the surface 

 between the umbones and the inferior margin is laterally flattened, 

 and covered with coarse prominent concentric ribs or growth- 

 lines, which vanish rather suddenly as they approach the lateral 

 margins. 



Q.J.G.S. No. 156. 2q 



