THE LAMELLIBRANCHIA 



275 



the organs ; the heart traversed by the rectum ; two aortae. The shell 



with a pallial sinus ; the dorsal region protected by accessory calcareous 



pieces. Genera PJiolas, Linnaeus ; foot cylindrical ; siphons free near 



their extremities ; British. In Pholas there are four accessory plates ; in 



Zirphaea, Leach, two ; in Barnea, Leach, one. Pholadidea, Goodall ; 



foot rudimentary ; siphons completely united and their extremity 



surrounded by a fringed disc ; shells elongated, in the adult prolonged 



posteriorly by a short calcareous tube, which surrounds the siphons > 



British. Jouannetia, des Moulins ; foot rudimentary ; 



siphons completely united ; shell globular, and the right 



valve prolonged posteriorly by a rostriform appendage. 



Xylophaga, Turton ; siphons separate at their extremities ; 



foot narrow ; shell globular with two accessory dorsal 



plates ; British. Martesia, Leach ; siphons long, united ; 



foot absent in the adult ; shell ovoid with a ventral plate 



in addition to the dorsal plates. FAMILY 2. TERE- 



DINIDAE, Fleming. Shell globular, covering a small 



portion only of the vermiform body. Heart on the 



ventral side of the rectum (Fig. 195, h) ; a single aorta; 



siphons long, united to a large extent and furnished with 



two posterior calcareous "pallets" (Fig. 247, II). Genera 



Teredo, Linnaeus ; a borer in wood ; secretes an 



adventitious non - adherent tube ; British. Xylotrya, 



Leach ; the pallets articulated. 



SUB-ORDER 9. ANATINACEA. 



Hermaphrodite Eulamellibranchia, in which the 

 ovaries and testes are distinct and have separate orifices 

 (Fig. 219, o, t). The foot generally rather small. The 

 mantle frequently presents a fourth orifice. The ex- 

 ternal gill-plate directed dorsally and devoid of a reflected 

 lamella. Hinge of shell without teeth. 



FAMILY 1. THRACIIDAE, Dall. Mantle with a fourth 

 pallial orifice ; the pedal orifice elongated ; 



Fio. 247. 



Teredo navalis, 

 Linnaeus, ventral 

 aspect. I, shell ; 

 II, pallets ; III, 

 anal 



siphon ; IV, 



siphons branchial siphon ; 

 ,., V, siphonal mass ; 



rather long, quite separate, and completely retractile vi, foot. 

 and invertible. Shell with a deep pallial sinus. 

 Genera Thracia, de Blainville ; shell with a large spoon-shaped tooth ; 

 British. AstJienothaerus, Carpenter ; shell without spoon-shaped teeth. 

 FAMILY 2. PERIPLOMIDAE, Dall. Siphons separate, naked, completely 

 retractile, but not invertible. Pallial sinus shallow ; no ligament. 

 Genera Cochlodesma, Couthouy. Periploma, Schumacher. Tyleria r 

 Adams. FAMILY 3. ANATINIDAE, Gray. Siphons long, united, covered 

 by a chitinous sheath, and not completely retractile. Foot slender. 

 Pallial sinus well marked. Genera Anatina, Lamarck. Shell thin and 

 gaping, with spoon-shaped teeth. Plectomya, de Loriol ; fossil from the 

 Jurassic and Cretaceous. FAMILY 4. PHOLADOMYIDAE, Gray. Mantle 

 extensively closed, with a fourth orifice. Siphons very long, completely 

 united, naked, and incompletely retractile. Foot small, with a posterior 

 appendage. Shell thin, with an external ligament and a well-marked 



