88 SUPPLEMENT TO GREAT OOLITE MOLLUSCA. 
forwards; the six or seven coste posterior to it are much smaller, they diminish 
regularly in prominence, are closely arranged, and are deeply indented by the regular, 
large, concentric plications; the anterior side gapes slightly, and has two inconspicuous, 
indented cost ; the posterior aperture is narrow and lengthened. 
The sub-conical figure, pointed posterior side, and large, carinated rib, will serve to 
distinguish it from P. Heraulti, Ag., to which it is nearly allied; the more angulated 
figure, and more numerous cost, from P. carinata, Ag. Some examples of Pholadomya 
from the Inferior Oolite are not distinguishable from P. /yrata; but between these and 
P. Heraulti are others, which apparently serve to connect the two forms, so that it is 
difficult to separate them altogether from P. /yrata, although undoubtedly they must be 
merged with P. Heraulti ; these connecting links are also quite irrespective of any changes 
that may be due to the stage of growth in either of the two species. 
Geological Positions and Localities. P. lyrata is common in the Cornbrash of 
Wiltshire and Dorsetshire. Dr. Oppel records it in the same rock at Marquise, near 
Boulogne, and Egg, near Aran, D’Orbigny (‘ Prodrome,’ i, p. 252) quotes it from the 
Upper Lias, near Bath, which is an error copied from the ‘Mineral Conchology of 
Great Britain.’ 
Homomya ecrpposa, Sow., sp. Part II, Tab. XII. fig. 14; Tab. XLIII, figs. 2, 2 a. 
Described at page 138, Part II, under the name of MJyacites gibbosus. As this shell 
occurs abundantly both in the Cornbrash and the Inferior Oolite of the southern counties 
of England, a full-sized average example is here figured ; occasionally, indeed, the species 
acquired much larger dimensions, as in the Cornbrash of Wiltshire, but it is then 
invariably more or less distorted and imperfect; it is also more gibbose than the smaller 
examples. Since the publication of the former portions of this Monograph more 
extended information respecting this and other allied species comprised in the proposed 
genus Homomya of Agassiz has led to the conclusion that they cannot be assigned to the 
genus J/yacites, of which they possess neither the external granulated tegument nor the 
peculiar characters of the hinge. 
When the surface of JZyacites has been denuded of the granulated tegument it is 
smooth, with irregular, longitudinal laminee, whereas Pholadomya and Homomya have a 
wrinkled or corrugated surface. 
The genus Homomya was intended by Agassiz to include shells whose forms 
resemble those of the more lengthened Pholadomyas, but which are destitute of radiating 
costa, and have usually a thicker test, the hinge being identical with that of Pholadomya, 
usually, indeed, more massive; but although the sides of the valves are destitute of 
coste, it occasionally happens that a few delicate, radiating lines, more or less obscurely 
marked, are visible upon the umbones, but vanish before they reach the middle of the 
