BIVALVIA. 101 



There yet remain a very numerous and varied series of the fossil Myadae, which have 

 been separated by Agassiz under the names of Myopsis, Pleuromya, Arcomya, Platymya, 

 and Homomya. These forms are found in the Muschelkalk, and throughout all the 

 Secondary rocks ; one or more species likewise occur in the older Tertiary rocks of England. 

 M. Agassiz believed that Myopsis was distinguished from the others by the presence of a 

 tooth in the hinge (Jide D'Orbigny), although he had never been able to detect its 

 presence, and also by its possessing a surface ornamented by radiating lines of granules. 

 We have been enabled to ascertain that the most abundant of the British Myopsides 

 (Mya dilata, Phil.) is destitute of any cardinal tooth, and that the granulated surface, 

 which M. Agassiz relied upon as distinguishing Myopsis, is possessed also in a manner 

 more or less modified by Arcomya, Platymya, Pleuromya, and Homomya. 



There remains, therefore, between these proposed genera little more than the distinction 

 of figure ; and even this feature, although sufficiently remarkable and distinct in certain 

 selected typical species, approximates so nearly in others, that in very many instances it is 

 only possible to separate them as distinct groups by an arbitrary and uncertain arrangement. 

 The test of these shells is very thin, and a depression more or less distinct exists upon the 

 anterior side of the valves, extending from the umbones to the inferior border. 



The Myopsides are usually elongated posteriorly ; their siphonal aperture is large, and 

 their radiating lines of granules are distantly arranged, and large upon the posterior side : 

 Mya dilata, Phil., is a well-known English example. Arcomya is more rhomboidal or 

 subquadrate ; the anterior side is compressed ; the posterior has an oblique prominence 

 the syphonal aperture is elongated and narrow. Pleuromya, with more tumid umboues, 

 has its superior border slightly concave, and the posterior third of the shell is attenuated 

 with a small aperture. Platymya resembles Myopsis, except that the umbones are placed 

 nearer to the middle of the valves ; the figure is more compressed, and the siphonal 

 aperture is small. Homomya resembles the more elongated of the Pholadomyas. The 

 umbones are large and usually but little compressed ; but, with this exception, there is 

 nothing to distinguish the figure from one or other of the preceding types, insomuch that 

 M. Agassiz, in the absence of a knowledge of the test, was sometimes unable to allocate 

 them to either of his proposed genera. Certain of the shells which Agassiz would refer to 

 Homomya possess a feature which tends to approximate them to the true Pholadomyas, 

 viz., the presence of a few faintly marked radiating costas upon the umbones. These, 

 however, are uncertain, and sometimes vanish altogether. Such species appear to form a 

 true passage, connecting the more elongated Pholadomyas with the Panopaeas. The 

 granulated surface which distinguishes this great series of fossil Myadse presents several 

 modifications of character, and tends to separate more fully the several species. The first 

 modification has the granules rather large, placed upon lines which are slightly elevated 

 and distantly arranged : some of the larger Myopsides and Arcomyae present examples. 

 The second modification has the lines of granules distinct ; but the granules are minute, 

 and both these and the rows are very closely arranged : the Pleuroniyee have usually this 



