LAMELLIBRANCHIATA. | LOWER PALAXOZOIC MOLLUSCA. 257 
distributes the blood. Nerves: in the Monomyaria the principal ganglion is on the ventral convex edge of the 
adductor, to which it sends branches, as well as to the gills and mantle; two yellow filaments extend to the 
mouth, where another pair of ganglia supply the tentacles, and give off a delicate ring round the cesophagus. 
In the Dimyaria the main pair of ganglia (“ branchial”) are at the under edge of the posterior adductor, 
sending, as before, cords to the supracesophageal ganglia, between the mouth and the anterior adductor ; 
numerous other small ganglia are developed in various positions. Hearing: Cyclas has at the anterior part of 
the labial tentacles a cavity, with a cretaceous, oscillating nucleus, considered by Dr Siebold to be the ear. 
Sight: Pecten has numerous small eyes round the edge of the mantle, but most of the types are blind. De- 
velopment; the spermatic filaments discharged from the males are drawn in by the branchial currents of the 
females. The young, or embryo, of the dimyarian genus Anodon have only a single adductor, as in the mono- 
myaria, and possess a byssus. The shells are extravascular, of an external cellular, or fibrous, and a thin 
internal nacreous layer forming the hinge teeth, Ge. 
The class is divisible into three Orders :—1st, Plewroconcha, with unequal valves, unsymmetrical bodies, resting 
habitually on one side, lobes of the mantle disunited and never forming siphonal tubes, and the pallial impression 
being consequently entire. 2nd, /sedrolotila, with equal valves and entire pallial sear. 3rd, Wacrotrachia, with 
equal valves and a sinus in the pallial scar, indicating large, retractile, siphonal tubes, The two latter are 
the most highly organized, and retain a vertical position when alive, as pointed out by d’Orbigny. The Ist and 
2nd groups are confounded under the name Atrachia by Swainson, whose group Macrotrachia contained all 
the other Lamellibranchs; M. d’Orbigny has distinguished the very natural group Plewreconcha, but confounds 
the 2nd and 3rd groups together under the name Orthoconcha. 
ist Ord. Prievroconcua (D’Orb.) 
Shell inequivalve, unsymmetrical. Animal unsymmetrical; lobes of the mantle disunited, never forming 
siphonal tubes ; pallial impression consequently entire. Natural position, resting on the side, 
These shells are inferior to the Jsedrolotila and Macrotrachia in general organization; they are not sym- 
metrical like them, but one valve differs in size, &c. from the other; and instead of being free and habitually 
assuming a vertical position, they lie on one side, and are fixed to foreign bodies either by a byssus, or by the 
substance of the lower valve ; the shells in the latter case being remarkable for the irregularity of their growth. 
This Order contains the following families: Ist, Anomiide ; 2nd, Ostreidw; 3rd, Etheride ; 4th, Cha- 
mide ; 5th, Spondylide ; 6th, Pectinidw ; 7th, Aviculide. 
7th Family. AVICULIDE. 
Shell very inequilateral and very inequivalve, obliquely elongate; the upper or left valve largest ; a notch for 
the byssus under the anterior ear of the lower valve ; pallial line entire; two muscular impressions in each valve, 
one large a little behind the middle, another very small and inconspicuous just under the beak (probably only a 
retractor of the foot) ; hinge linear, with or without teeth ; cartilage internal or external, simple or transversely 
divided. Animal: mantle quite open, margin thick and ciliated; foot rudimentry, cylindrical, beneath which is 
a very large byssus for attachment; gills forming two crescent-shaped plates on each side; mouth with wide 
truncated lips. 
The shells of this family differ from those of the family Pectinidew by the obliquity of the shell, and by the 
structure of the hinge and ligament. They abound chiefly in the warm seas of the Tropics, 
Genera.—I\st, Avicula ; 2nd, Pterinea ; 3rd, Pteronites; 4th, Aucella; 5th, Ambonychia; 6th, Streblopteria; 
7th, Aviculopecten; 8th, Gervillia; 9th, Perna ; 10th, Inoceramus; 11th, Pulvinites. 
Genus. AVICULA (K7ein.) 
Gen. Char.—V ery inequivalve and inequilateral, obliquely oval; hinge produced posteriorly into a flattened 
defined Wing; the inferior or right valve flattened, notched for the passage of the byssus; anterior (? retractor) 
muscular impression very small and faintly marked; adductor large, superficial, a little behind the middle; 
[Fasc. 11.] Lu 
