602 MANUAL OF THE MOLLUSCA. 



Synonyms, Byssomya, Cuv. Ehomboides, Bl. Hiatella 

 (minuta), Daud., Biapholius, Leach. Arcinella (carinata), Phil. 



Shell when young symmetrical, with 2 minute teeth in each 

 valve ; adult rugose, toothless ; oblong, equivalye, gaping, 

 ligament external ; pallial line sinuated, not continuous. 



Animal with mantle-lobes united and thickened in front ; 

 siphons large, united nearly to their ends, orifices fringed ; 

 pedal opening small, foot finger-like, with a byssal groove ; 

 palpi small, free ; gills narrow, unequal, united behind and 

 prolonged into the branchial siphon. 



Five genera and 15 species have been manufactured out of 

 varieties and conditions of this Protean shell. It is found in 

 crevices of rocks and corals, and amongst the roots of sea-weed, 

 or burrowing in limestone and shells ; at Harwich it bores in 

 the cement stone (clay iron-stone), at Folkestone in the Kentish- 

 rag, and the Portland stone employed in the Plymouth Break- 

 water has been much wasted by it. Its crypts are sometimes 

 6 inches deep (Couch) ; they are not quite symmetrical, and 

 like those of the Lithodomics, are inclined at various angles, so 

 as to invade one another, the last comers cutting quite through 

 their neighbours ; they are usually fixed by the byssus to a 

 small projection from the side of the cell. The Saxicava ranges 

 from low water to 140 fathoms ; it is found in the Arctic 

 Seas, where it attains its largest size ; in the Mediterranean, 

 at the Canaries, and the Cape. It occurs fossil in the Miocene 

 tertiary of Europe and in the United States, and in all the 

 glacial deposits. 



Clavagella, Lamarck. 



Example, 0. bacillaris, PL XXIIL, Pig. 17. 



Shell oblong, valves, flat, often irregular or rudimentary^ 

 the left cemented to the side of the burrow, when adult, the 

 right always free; anterior muscular impression small, posterior 

 large, pallial line deeply sinuated. Tube cylindrical, more or 

 less elongated, sometimes divided by a longitudinal partition ; 

 often furnished with a succession of siphonal fringes above, 

 and terminating below in a disk, with a minute central fissure, 

 and bordered with branching tubuli. 



Animal with the mantle closed in front, except a minute slit 

 for the foot, and furnished with tentacular processes ; palpi long 

 and slender ; gills 2 on each side, elongated, narrow (fioating 

 freely in the branchial siphon ?). 



Some specimens of the recent 0. aperta have 3 frills to their 

 tubes, C. bacillaris has twice that number occasionally. They 



