136 CLASS I. TROPIOPODA. ORDER II. UNIMUSCULOSA. 



prominulis. Cardo linearis, ligamento marginali, maxime interno. 

 Impressio muscularis composita, indistincta. Animal bysso nullo ? 



The genus Lithodomus was introduced by Cuvier for the purpose of 

 distinguishing a small group of terebrating mollusks, previously included 

 with the Modioli. From a certain resemblance in their shell to that of 

 Modiola, Lamarck, and even some authors of the present day, consider 

 this distinction unnecessary ; but when we find two animals differing so 

 entirely in their habits, the one living in the sea, attached by a byssus to 

 submarine rocks, the other without occasion for a byssus, dwelling in 

 concealed cavities of rocks resulting from its own mechanical contri- 

 vance ; is it not probable that nature has given to each a corresponding 

 and peculiar system of organization ? 



The Lithodomi are evidently terebrating animals, being found buried 

 in stone, madrepores, &c, in all their different stages of growth : their 

 shell may be described as being transverse, cylindrically oblong, equi- 

 valve, and covered with a brown epidermis ; the extremities, the anterior 

 of which is much the shorter, are round ; and the umbones are scarcely 

 prominent. The hinge is linear, having a long marginal ligament, which 

 is mostly internal. The muscular impression is compound and rather 

 indistinct, and the animal does not appear to be provided with any 

 byssus*. 



Examples. 



PI. XCIX. Fig. 1 and 2. 



Lithodomus dactylus, Cuvier, Regne Animal, vol. ii. p. 471. Enc. 

 Me'th., pi. 221. f. 0, 7. Sowerby, Genera of Shells, No. 23. 



Both Cuvier and Poli distinctly assert that the Lithodomi are provided with a hyssus when 

 young, and that previous to their operation of boring they adhere to the rocks like the rest of 

 the Mytilacea. This assertion has, however, been contested by Sowerby, without reference to 

 the animal, upon the following argument, — that it is contrary to the nature of an animal to 

 be at one time attached by a byssus and not at another. For our own part (though fully sen- 

 sible of the caprice of Nature), we are strongly inclined to support the reasonableness of this 

 opinion ; and when it is a known fact, that the Lithodomi are as commonly found buried in 

 stone in the very young as in the adult state, what further confirmation can be required ? 



