FAMILY 2. MYTILACEA. 137 



Mytilus lithophagus, Linnaeus. De Blainville. 

 Modiola lithophaga, Lamarck. 

 Lithodomus lithophagus, Payrandeau. 



PL XCIX. Fig. 3 and 4. 



Lithodomus caudigerus, Sowerby, Genera of Shells, No. 23. Enc. 



Me'th., pi. 221. f. 8. 

 Mytilus aristatus, Solander. 

 Mytilus caudigerus, Deshayes. 

 Modiola caudigera, Lamarck. 



PI. XCIX. Fig. 5. 

 (An undescribed fossil species in a Madrepore from the great Oolite.) 



MODIOLA, Lamarck. 



Testa oblonga, aequivalvis, regularis, imequilaterahs, latere antico brevis- 

 simo, obtuso, hiantulo. Cardo linearis ligamento marginali, sub- 

 interno. Impressio muscularis composita, sublateralis. 



The Lithodomi, the Modiolae, and the Mytili are united both by De 

 Blainville and Deshayes in one and the same genus. Lamarck unites 

 the Lithodomi and the Modioli ; but, for our own part, we consider that 

 a much stronger affinity exists between the Modiolae and the Mytili ; and 

 if it had been advantageous to reduce the number of genera into which 

 the Mytilacea are divided, we should certainly have rather decided on 

 suppressing the genus under consideration. The Modiolse entirely re- 

 semble the Mytili in their habits, and their shells differ only in the short 

 obtuse termination of the anterior side ; still, although they pass very 

 nearly into the depressed longitudinally-triangular form of the latter, 

 they may, nevertheless, be separated from them with tolerable accuracy. 

 The shells of the Modiolae are well known ; and when such is the case, 



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