FAMILY 2. MYTILACEA. 141 



PINNA auctorum. 



Testa requivalvis, obliqua, cuneiformis, longitudinalis, fibro-laminaris, ex- 

 tus aut muricata, aut complanata ; umboaibus acutis, terminalibus ; 

 lateribus, antico subbiantulo, postico sa?pe valde hiante. Cardo late- 

 ralis, edentulus, ligamento subinterno per totam longitudinem con- 

 tinue Impressio muscularis composita. 



It is difficult to trace the precise origin of the present genus ; the 

 animals which it represents having been associated together in the natu- 

 ral system long before the time of Linnaeus. They appear, indeed, to 

 have been well known to the ancients, and are described under the 

 title of Ulvva by Aristotle, Oppian, Phyle, and other early historians. 

 Although somewhat allied to the Mytili, they may be readily distin- 

 guished by the large size, and prismatic crystalline texture of their 

 shells ; they differ also in having them often fretted with tubercles or 

 blunted spines. What we have in the course of our descriptions of the 

 Mytilacea termed the compound construction of the adductor muscle is 

 especially marked in these animals ; in fact, Poli, the celebrated Italian 

 conchologist, who possessed great local advantages for examining the 

 Pinnae of the Mediterranean, has published an elaborate description of 

 their anatomy, in which he asserts that there are two distinct muscles 

 of attachment (vide PI. CIV.). Still he admits that one is exceedingly 

 small ; we may therefore consider it (in accordance with the opinion 

 of Lamarck, and without injury to the basis of our primary division of 

 the Tropiopoda) as an accessory cartilage destined to assist the chief 

 muscle in counteracting the powerful expanding action of the hinge 

 ligament ; we term it a compound muscle too, as analogous to the com- 

 pound anterior muscle of some of the Bimusculosa. The genus Pinna is 

 one of considerable interest, because we meet with a new and particular 

 structure in the composition of the shell. Instead of being solid and 



