MYOPHOLAS. 253 
PHOLADOMYA CORDATA, J'ate, 1865. Plate XLII, fig. 2 a—e. 
1865. PHonapomya corpata, R. Tate. Quart. Journ. Geol. Soc., vol. xxi, p. 40, 
pl. iv, fig. 1. 
Remarks.—A specimen found in the Upper Chalk (zone of Belemnitella mucro- 
nata), now in the Norwich Museum (No. 3339), is probably an example of P. 
cordata, but unfortunately the anterior part of the shell is imperfect. In this 
species the anterior curvature of the umbones is greater and the depression in 
front of them deeper than in P. decussata; also the anterior flattened part is 
relatively smaller, since the greatest diameter of the shell occurs at about one third 
of the length from the anterior end. The types of P. cordata from the Upper 
Chalk of Ireland are in the Museum of Practical Geology (Nos. 23,628 ; 25,629). 
Genus—Myornonas, H. Douvillé, 1907. 
(‘ Bull. Soe. géol. de France,’ ser. 4, vol. vii, p. 107.) 
5 
Myopuonas, sp. cf. semicosrara (Agassiz), 1842. Plate XLII, fig. 3a, bd. 
Description.—Shell oval, short, considerably inequilateral, anterior part convex, 
posterior part compressed; anterior and posterior margins rounded. Umbones 
curved inwards and forwards. Rather more than half of the shell—the anterior 
part—is ornamented with thirteen narrow, sharp, radial ribs, which are separated 
by broad, flat, or shghtly concave interspaces; posteriorly these ribs become 
smaller and less widely separated; on the middle part of the shell a few indistinct 
concentric ribs are seen. The posterior part of the shell is nearly smooth, and is 
separated from the ribbed area by a slight depression; a curved carina extends 
from the posterior side of the umbo towards the postero-ventral extremity. 
Affinities.—Only one specimen has been seen; it resembles closely M. seivi- 
costata (Agassiz'), but is rather shorter, and the posterior limit of the ribbed area 
is more sharply defined than in most examples of M. semicostata. Judging from 
1 «Etudes crit. Moll. Foss.,’ Myes (1842), p. 51, pl. ii, figs. 1, 2, pl. ui}, fig. 11. Pictet and 
Campiche, ‘ Foss. Terr. Crét. Ste. Croix’ (‘ Matér. Pal. Suisse,’ ser. 4, 1865), p. 77, pl. ev, figs. 1, 2. 
Moesch, ‘Mon. Pholadomyen’ (1874), p. 85, pl. xxx, fig. 4, pl. xxxiii, figs. 3, 4, pl. xxxvi, fig. 1. 
Douvillé, ‘ Bull. Soc. géeol. de France,’ ser. 4, vol. vii (1907), p. 112, pl. ii, fig. 8. Moesch includes as 
a synonym Pholadomya Triboleti, Pictet and Campiche, op. cit., p. 89, pl. evi, fig. 8. Pictet and 
Campiche include P. Moreana, Buvignier, ‘ Statist. géol., ete., de la Meuse ’ (1852), Atlas, p. 8, pl. viii, 
figs. 21, 22. 
