OP SOUTHERN INDIA. 365 



the single name Mytilus, there cannot be, however, any doubt. They represent 

 three well-marked types of shells. 



Another question arises as to whether it be more correct to unite the three 

 sub-families just pointed out into one family, or to divide them into two or even 

 three. The sub-division of the Mytilidm which I have here adopted is mainly 

 based upon the development of the in- and ex-halant openings. As regards this 

 point of organisation the Mytilibm offer a most marked parallel to the Caudiidm, 

 In both we find that certain marine forms exhibit a gradual transition into others 

 inhabiting brackish and fresh water, and that while there is but very little change 

 in the general form and structure of the shell, certain organs of the animals attain 

 a higher development ; namely, the simple siphonal folds of the mantle become 

 first slightly and then more markedly prolonged into tubes, and at the same 

 time the mantle margins grow together. Upon this distinction the separation of 

 the marine cardiinje and the fluviatile lymnocardiinm has been based. Now, there 

 is actually little difference between the purely marine mytilinm and the partially 

 brackish, partially marine crbnellin^^ both possessing an open branchial mantle- 

 fold, and the former a simple enclosed anal slit, the latter a regular more or less 

 prolonged anal tube ; both have, as a rule, the mantle margins disunited, only 

 Crenella appears to have the ventral margins partially united. This form, there- 

 fore, clearly indicates a passage to the brjeissenin^, the typical forms of which 

 have the mantle closed, though it is not yet certain whether the same character 

 applies to the other recent genus of the sub-family, Septifer (the animal of which 

 is believed not to differ from Mytilus), Looking thus at the relations of the ani- 

 mals, in which a gradual change of certain organs can be traced, and considering 

 at the same tim.e the very close affinity of the shells, I do not think that a separa- 

 tion of the DRJEissBNiN^ iuto a distinct family from the Mytilidm can justly be 

 considered as natural in the present state of our knowledge. Much less do I 

 believe in the close affinities of the genus Breissena to Sphcerium fCyclasJ, or to 

 Cardita, an opinion which has lately found favor among some European concho- 

 logists. 



The Mytilid^ class among the oldest Pelecypoda, They already appear in the 

 palseozoic strata, and continue in increasing variety and number of species up to 

 the present date. 



a. Suh-famili/~DBEIS8ENIN^, 



Anterior muscular scar resting on a thickened plate near the apex of the 

 shell, hinge sometimes with an obsolete long tooth, fulcra of ligament strong; 

 pallial line entire, rarely sinuated. 



1. JSoplomytihts, Sandberger, 1856, (Verst. des Ehein. Schichtensystems 

 in Nassau, p. 281). Sub-triangular, with concentric striae of growth; beaks 

 pointed, an elongated plate extends from below the beak, where it is attached to the 

 anterior side and for some distance along the dorsal margin ; next to it lies a rib-like 

 tooth in the right valve corresponding to a groove in the left one, and then follows 



