502 MANUAL OF THE MOLLUSCA. 
Synonyms, Byssomya, Cuy. Rhomboides, Bl. Hiatella 
(minuta), Daud., Biapholius, Leach. Arcinella (carinata), Phil. 
Shell when young symmetrical, with 2 minute teeth in each 
valye; 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 grooye ; 
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 Lithodomus, 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, C. bacillaris, Pl. X XIII., Fig. 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 (floating 
freely in the branchial siphon f). 
Some specimens of the recent C. aperta have 3 frills to their 
tubes, C. bacillaris has twice that number occasionally. They 
