On the Ligament of Bhuhcs. 217 



edge, tliere is less and less of this latter substance deposited, so 

 that at the end the plates are only formed of the animal matter. 

 Now as these plates are deposited in a semi-fluid state, why should 

 not their edges be soldered together, and form this external coat ; 

 for we find that it is formed of concentric striae, similar to the 

 striae of growth, and that it separates more readily at these striae 

 than elsewhere ? 



This substance is formed at the external part of most shells, but 

 varies very much in outwai;d appearance ; it thus oifers very good 

 characters for the distinction of Families ; being sometimes very 

 thick, as in the Unionidw, at others very thin ; and sometimes, as 

 in the Cypraeae, being covered by the reflexion of the inner surface 

 of the shell over it. The Oliva;, Marginellae, &c. appear to be desti- 

 tute of it altogether, as well as those few shells that are con- 

 stantly covered by the lobes of the mantle being soldered over 

 them. This Epidermis is formed on the external part of the 

 shelly operculum^ and we may perhaps say that the horny oper- 

 cula are entirely formed of the same kind of substance as the 

 Periostraca. 



% 10. On the " LigamenV of Bivalves. 



Perhaps there is no part of the shell which has been so much 

 misunderstood as the ligament ; some have considered it as pos- 

 terior, others as anterior; and some, as Lamarck and Burrows, 

 sometimes on one side and sometimes on the other; some have 

 made it dorsal and others ventral. But the position is not the 

 only source of error, for its functions and structure have been 

 equally misunderstood ; thus most persons have evidently con- 

 sidered it as an homogeneous substance. Lamarck partly (thougli 

 unconsciously), corrected this error, but he did not arrive at the 

 truth, for he divided his Mactraces into those that have only 

 an internal, and those which have an external and internal liga- 

 inent ; but 1 do not know what he could correctly place in 

 the former section, for the shells which he refers to it, have 

 as distinctly an external ligament as the others. lie committed 

 JiowevcT a still greater error respecting the fuuclionu of tliis 



Vol.. I. p 



