88 SUPPLEMENT TO GREAT OOLITE MOLLUSCA. 



forwards ; the six or seven costse 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 costse ; 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 costaj, from P. carinata, Ag. Some examples of Pholadomya 

 from the Inferior Oolite are not distinguishable from P. lyrata ; 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. lyrata, 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 GiBBOSA, Sow., sp. Part II, Tab. XII. fig. 14; Tab. XLIII, figs. 2, 2 a. 



Described at page ]38, Part II, under the name of Myacites gihhosus. 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 Myacites, of which they possess neither the external granulated tegument nor the 

 peculiar characters of the hinge. 



When the surface of Myacites has been denuded of the granulated tegument it is 

 smooth, with irregular, longitudinal laminae, 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 

 costse, 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 

 costaj, it occasionally happens that a few delicate, radiating lines, more or less obecurely 

 marked, are visible upon the umbones, but vanish before they reach the middle of the 



