MYTILID^. 163 



in Lithodomus the external stratum has a remarkable 

 tubular structure, resembling that of dentine. The shell of 

 Dreissena has a verj different organization. In it the in- 

 ternal layer is composed of large prismatic cells. Between 

 the internal white layer and the periostracum is a layer 

 of a brownish colour, in which also traces of a cellular 

 structure present themselves ; especially in a section made 

 perpendicularly to its surface, which brings into view a de- 

 cided prismatic arrangement.* Such marked differences of 

 minute structure in genera so very closely and unquestion- 

 ably allied, leads us to infer that they afford but feeble 

 indications of the true position or rank of a species or group 

 in the series, though probably important as indications of 

 the special adaptation of the creature under examination to 

 peculiar conditions. 



In the oldest fossiliferous rocks yet explored we find the 

 remains of bivalve shells presenting striking resemblances, 

 and indicating close affinities to the Mytilidce now existing. 

 As, however, in the latter, form of shell affords but a feeble 

 clue to the distinctions which separate genera, it is by no 

 means improbable that several groups of generic value, 

 long since become extinct, may have existed in ancient 

 epochs, though the shapes of their shelly envelopes are so 

 very similar. 



DREISSENA. Van Beneden. 



Shell equivalve, very inequilateral, sub-triangular, 

 tumid, surface covered Avith an epidermis, beaks terminal, 

 furnished internally with a transverse shelf or partition. 

 Hinge composed of an imperfectly developed cardinal tooth 

 in the right valve, and a corresponding socket in the left : 



* Brit. Assoc. Report. 1847. 



