102 MOLLUSCA FROM THE GREAT OOLITE. 



kind of surface. The third modification has the entire surface covered with granules so 

 minute as to be nearly or altogether invisible to the unassisted eye. They are so dense 

 that the linear arrangement cannot be recognised : the Homomyge have this kind of surface. 

 In the present state of our knowledge, it would not appear that the figure of the 

 shell affords any certain guide to the character of the granules which adorn its surface, a 

 general resemblance of form being sometimes coincident with a very different kind of 

 surface, and in the fossil Myadae, wherein the figures of the individuals present much 

 variability, and consist more commonly only of casts, the presence of a small portion of 

 the outer granulated tegument will in some instances serve as a sure guide to distinguish 

 species for which the casts alone would not have sufficed. It is owing to the absence of 

 the test that so many of the figures of the 'Etudes Critiques' of Agassiz afford only doubtful 

 guides to the correct knowledge of the species. 



The foregoing observations will prepare the reader for the conclusion at which we have 

 arrived, viz., that Myopsis, Pleuromya, Arcomya, Platymya, and Homomya cannot claim 

 to be regarded as distinct genera, and that it is very difficult, or perhaps not practically 

 possible, even to separate them into so many sections or sub-genera. They seem rather to 

 constitute a single very extensive and varied series of forms, which, although individually 

 resembling in certain of their features either Pholadomya or Panopaea, are nevertheless 

 sufficiently separated from both of these genera, and possess a generic entirety which is 

 rather strengthened than otherwise by these resemblances. 



The hinge exactly resembles that of Pholadomya, except that the subligamental lamina 

 is more stout, and the test at that part of the shell is generally more thickened. It is 

 therefore destitute of the sharp tooth of Panopaea ; but even this feature is not without 

 exceptions, for M. Buvignier has figured an Oolitic species, which has a distinct tooth, and 

 we have ourselves discovered a tooth slightly defined in an Inferior Oolite shell, other 

 examples of which present no trace of this feature. 



The hinge then generally resembles that of Pholadomya, and some few species or rather 

 individuals of these species, by possessing a few delicate radiating costae upon the umbones, 

 present another feature which tends to approximate them to the same genus. To 

 Panopaea other examples are allied by the occasional presence of a projecting cardinal tooth, 

 and by a universal flattening or depression upon the anterior third of the valves. The 

 granulated surface, however, removes it equally from Panopaea and Pholadomya. In the 

 figure of the muscular impressions we recognise a close resemblance to those of Pholadomya, 

 the anterior impression being very narrow, pyriform, and so much elongated upwards as to 

 reach nearly to the umbo. In Panopaea the figure of this impression is irregular and 

 different. The siphonal flexure is always very great, whatever may be the figure of the 

 posterior side of the shell. Briefly to recapitulate these analogies and differences : our 

 group is allied to Pholadomya, in the features of the hinge and of the muscular im- 

 pressions, but differs from it in the absence of costae, in the presence of radiating lines of 

 granules upon the surface, and in the vertical depressions upon the sides of the shell. It 



