66 AKCAD^E. 



The family of the Arcada is the first, according to our 

 method, in which the adductor muscles are established in 

 their pyriform shape, similar size, and lateral position : the 

 departure from the muscular disposition of the Ostreadoe, to 

 the lateral one of the great mass of the bivalves, commenced 

 in Mytilus, and has become complete in this family. 



Since the above was written we have added to this family a 

 sixth genus, Lepton, under which head our reasons for this 

 procedure will be found. 



ARCA, Linnseus. 



A. TETEAGONA, Poli et Auct. 

 A. tetragona, Brit. Moll. ii. p. 234, pi. 45. f. 9, 10; (animal) pi. P. f. 1. 



Animal elongated, thick, white; mantle open throughout 

 the ventral range ; its edge is plain, but the upper part of the 

 posterior margin is marked, as in the Pectunculi and Pectines, 

 with about 40 close-set, equidistant dark dots or ocelli, which 

 become obsolete and vanish anteriorly ; the margin, or area 

 within the ocelli, is mottled with pale yellow flake markings, 

 which are also seen, though more faintly, on the under sur- 

 face. The foot is white, and when at rest is bent, but, ex- 

 serted, tapers subconically. There is a deep byssal groove at 

 the posterior geniculated portion, for the green filamentous 

 mass, which at the point of insertion into the cavity is split 

 into two laminae; a neighbouring gland appears to supply a 

 green glutinous fluid, from which the filaments are formed ; 

 we conclude so, as whenever the byssus was extracted, the 

 animal immediately produced from the fissure a flow of matter 

 which almost instantly resolved itself into byssal threads ; and 

 it is quite certain it can free itself from a state of fixity, which 

 is effected by drawing the byssus from the groove, and entirely 

 deserting it. This mano3uvre we have also seen performed by 

 the animals of Area lactea and Galeomma Turtoni. There is 

 little doubt that all byssus-bearing mollusca, when not con- 

 fined in stony cavities, have the power of a limited locomotion 

 by abandoning their moorings. 



The foot, independent of the deep fissure at the base for the 



