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. FAMILY 8. MYALINIDAE, Freeh. 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. Hoplomytilus, Sandberger ; Devonian. PtycJio- 

 desma, Hall ; Devonian. Anthracoptera, Salter ; Carboniferous. Per- 

 gamidea, Bittner ; Trias. Mysidca, Bittner ; Trias. Aucella, Kyser ; 

 Jurassic and Cretaceous. FAMILY 9. AMUSSIIDAE, Ridewood. Gills with- 

 out interlamellar junctions. Shell orbicular, smooth externally, with 

 radiating costae internally. Genus Amussium, Klein. FAMILY 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. FAMILY 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 Pecteii, Lamarck ; shell orbicular, 

 with equal auriculae ; without a by.sal sinus ; British. Chlamys, Bolten ; 

 shell higher than it is long ; the anterior auricula the larger, and pro- 

 vided with a byssal sinus ; British. Pedum, Bruguiere. Hinnites, 

 Defrance. Pseudamussium, Adams. Camptonedes, Agassiz. Hyalopecten, 

 Verrill ; abyssal. 



SUB-ORDER 5. DIMYACEA. 



Dimyarian Lamellibranchia with an orbicular and almost equilateral 

 shell ; adherent ; the hinge without teeth and the ligament internal. 

 Gills with free non-reflected filaments. 



Family DIMYIDAE, Dall ; with the characters of the sub-order. 

 Genus Dimya, Renault ; recent, in abyssal depths, and fossil since the 

 Jurassic. 



ORDER 3. Eulamellibranchia. 



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. 



