PERNA. 



TESTA subaequivalvis^ complanata, lamellosa, 

 subirregularis. Cardo linearis, marginalis, sulcis 

 plurimis, transversis, parallelis, oppositis, liga- 

 mentum multipartitum inter se excipientibus. 

 Sinus byssi posticus, subhians, infra cardinis ex- 

 tremitatem, parietibus incrassatis. 



As we have already stated the illustrious Linne has arranged 

 the shells of this Genus among his Ostrece^ instead of dis- 

 tinguishing them as a particular Genus, as he might have 

 done, in the opinion of Lamarck, not only on account of 

 the peculiarities of the hinge, but also on account of the 

 mode by which they attach themselves to the submarine 

 rocks. In these particulars, Perna is unquestionably very 

 distinct from Ostrece; but on account of their general ex- 

 ternal similarity we are not astonished at Linne 's asso- 

 ciating them together, particularly when we consider that 

 he had not the means of descending to minutioe ; we 

 are rather surprized, that, with the very small means he 

 possessed, he should have laid so good a foundation, 

 and that succeeding Conchologists with their extended op- 

 portunities, have only added to and embellished the super- 

 structure. Lamarck has, however, described perfectly well 

 the characters essential to the Perna, not only to distin- 

 guish them from the Ostrece^ but also from the Crenatulce 

 and the Inocera?ni^ as well as from the Arcaceee, which, on 

 account of their numerous teeth, might have been con- 

 founded with them : these latter are, however, regular 

 shells, and the teeth of one valve are alternately received 

 between those of the other valve; while in the Pernte, the 

 teeth (if the interstices between the sulci may be so called) 

 lie one against another when the valves are closed : the 

 ligament also lies between the teeth in Perna, but in the 

 Arcacece is completely external. There is one expression 

 which Lamarck has made use of in his definition of Perna, 

 which, however^ he has virtually corrected in his observa» 



