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Order PEOLADACEA. 

 Family FIIOLABIDJE. 



Animal clavate, with a large truncated foot protruded through the otherwise 

 closed mantle; siphons elongated, connected nearly to their ends, and not 

 provided with shelly styles. Gills narrow, attached, closing the branchial 

 chamber ; palpi elongate. 



Shell always present, its valves generally protected by one or more accessory 

 dorsal plates. 



Inhabiting exaavations in wood or stone, the walls of which are sometimes, 

 but not frequently, lined with a testaceous deposit. 



Subfamily 1. Pholadinje. The valves with a gap anteriorly, which is never 

 closed in the adult shell. 



Subfamily 2. Jouannetin^e. Anterior ventral hiatus open in the young shell, 

 but invariably closed in the adult by a callous plate. 



Family TEREDIDJE. 



Animal elongate, subcylindrical, siphons united nearly to the end, their ex- 

 tremities armed with two shelly styles; foot long and narrow, protruded 

 through the united mantle lobes, which are thickened in front. Gills long ; 

 mouth with palpi. Shell, when present, globular, tripartite, included with the 

 animal in a more or less cylindrical testaceous tube, the siphonal end of which 

 is divided into two by a longitudinal partition. 



Subfamily 1. Teredin.e. Valves present, free, contained in the tube, which 

 is irregularly cylindrical, sometimes much contorted. Perforating timber. 



Subfamily 2. Teredinin^;. Valves with an accessory anterior dorsal plate : 

 their margins prolonged into a shelly tube when adult. Tube frequently con- 

 camerated ; siphonal extremity often truncate, and the opening contracted by 

 a six-lobed internal margin, (fossil.') 



Subfamily 3. Kuphin.e. Without valves. Tube clavately cylindrical, sunk 

 horizontally in sand. Never penetrating timber. 



The present paper will comprise the family Pholadida? as here limited, while 

 Teredidae will form the subject of a future article. 



Synopsis of Genera. 



Subfamily PHOLADINiE. 



Anterior hiatus always open. 



* With two dorsal accessory valves. 



Dorsal valves placed anterior and posterior to the beaks, 

 the anterior lanceolate, the posterior small, transverse. 

 Umbonal processes reflected over the beaks, closely applied. 

 Shell elongate Genus Pholas, Linn. 



Dorsal valves lanceolate, placed side by side. Umbonal 

 processes reflexed over the beaks, cellular beneath. Shell 

 oblong, ovate Genus Dactylwa, Gray. 



Dorsal valves half ovate, diverging, small. Umbonal 

 processes none, but the anterior margins of the valves re- 

 flexed. Shell globose Genus Xylophaga, Turton. 



Dorsal valves moderate, diverging ; anterior hiatus 

 small. Shell oblong-ovate, with a pair of siphonal valves 

 at their posterior end Genus Talona, Gray. 



