262 THE LAMELLIBRANCHIA 
cardinal teeth and two posterior oblique lateral teeth. Dimyarian, the 
anterior adductor being very small. An extinct family from the Silurian 
and Devonian. Genera—Ambonychia, Hall; Silurian.  Byssonychia, 
Ulrich ; Silurian.  Gosseletia, Barrois ; Devonian. Clionychia, Ulrich ; 
Silurian. Famity 8. Myauinipag, Frech. Shell very inequilateral, the 
posterior part greatly enlarged ; the umbones anterior or terminal ; the 
hinge straight, without teeth ; adductors subequal. An extinct family 
from the Silurian to the Cretaceous. Genera—Myalina, de Koninck ; 
Silurian and Devonian. Hoplonvtilus, Sandberger ; Devonian. Ptycho- 
desma, Hall; Devonian. Anthracoptera, Salter; Carboniferous. Per- 
gamidea, Bittner; Trias. Mysidea, Bittner; Trias. Aucella, Kyser ; 
Jurassic and Cretaceous. Famity 9. AmussIIDAE, Ridewood. Gills with- 
out interlamellar junctions. Shell orbicular, smooth externally, with 
radiating costae internally. Genus—Amussiwm, Klein. Faminy 10. 
SPONDYLIDAE, Fleming. Shell very inequivalve, fixed by the right valve, 
which is larger than the left. The ligament elongated in a transverse 
direction. No byssus. Genera—Spondylus, Linnaeus ; shell with spiny 
ribs, and adherent by the spines. Plicatula, Lamarck ; shell folded, 
adherent by the umbo of the right valve. Faminy 11. PECTINIDAE, 
Lamarck. Shell ornamented with radiating ribs; the dorsal border 
provided with two auriculae. Foot byssiferous. Mantle borders pro- 
vided with eyes (Fig. 235). Genera—Pecten, Lamarck ; shell orbicular, 
with equal auriculae ; without a byssal sinus; British. Chlamys, Bolten ; 
shell higher than it is long; the anterior auricula the larger, and pro- 
vided with a byssal sinus; British. Pedwm, Bruguicre. Hinnites, 
Defrance. Pseudamussium, Adams. Camptonectes, Agassiz. Hyalopecten, 
Verrill ; abyssal. 
Sus-OrDER 5. DIMYACEA. 
Dimyarian Lamellibranchia with an orbicular and almost equilateral 
shell; adherent ; the hinge without teeth and the lgament internal. 
Gills with free non-reflected filaments. 
Family Driuyipar, Dall; with the characters of the sub-order. 
Genus—Dimya, Ronault ; recent, in abyssal depths, and fossil since the 
Jurassic. 
ORDER 3. Hulamellibranchia. 
Lamellibranchia in which the edges of the mantle are generally 
united by one or two sutures (Figs. 221, 241, etc.). Two adductor 
muscles are usually present (Figs. 238, 241, 242, etc.). In the gills 
the branchial filaments are united at regular intervals by vascular 
junctions which transform the linear interfilamentar spaces into a 
series of fenestrae (Fig. 237). Similarly the lamellae of each gill- 
plate have vascular junctions which form afferent vessels in the 
interior of the plates. The gonads always have their own proper 
external orifices. The order comprises the following nine sub-orders : 
—Ostraeacea, Submytilacea, Tellinacea, Veneracea, Cardiacea, 
Chamacea, Myacea, Adesmacea, Anatinacea, 
