422 MANUAL OF THE MOLLUSCA. 
Distribution, 70 species, chiefly tropical; WM. modiolus, Arctic 
seas — Britain. 
Fossil, 150 species. Silurian? Lias —. United States, 
Kurope, Thibet, South India. 
Sub-genera. Lithodomus, Cuy. M. lithophaga, Pl. XVIL, 
Fig. 7. Shell cylindrical, inflated in front, wedge-shaped behind; 
epidermis thick and dark; interior nacreous.* Distribution, 
40 species. West Indies — New Zealand. Jossil, 35 species. 
Carb. —. Europe, United States. The ‘‘date-shell” bores 
into corals, shells (Fig. 25, p. 34), and the hardest limestone 
rocks ; its burrows are shaped like the shell, and do not admit 
of free rotatory motion. The animal, which is eaten in the 
Mediterranean, is like a common mussel; in L. patagonicus the 
siphons are produced. Like other burrowing shell-fish, they 
are luminous. Perforations of ZLithodomi in limestone cliffs, 
and in the columns of the Temple of Serapis at Puteoli, have 
afforded conclusive evidence of changes in the level of sea- 
coasts in modern times. (Lyeil’s ‘‘ Principles of Geology.”) 
Crenella, Brown. O. discors, Pl. XVII., Fig. 8. (Lanistes, 
Sw. Modiolaria, Beck.) S/el/ short and tumid, partly smooth, 
and partly ornamented with radiating strize; hinge-margin 
crenulated behind the ligament; interior brilliantly nacreous. 
Animal with the anal tube and branchial margins prominent. 
Distribution, 24 species. Temperate and arctic seas; Nova 
Zembla, Ochotsk, Britain, New Zealand. Low water — 40 
fathoms. Spinning a nest, or hiding amongst the roots of sea- 
weed and corallines. MM. marmorata, Forbes, burrows in the 
test of Ascidia. Fossil, 12 species. Upper Greensand —. 
Europe. : 
Modiolarca (trapezina), Gray ; Falkland Islands — Kerguelen, 
attached to floating sea-weed ; mantle-lobes united, pedal open- 
ing small, foot with an expanded sole, front adductor round. 
M. ? pelagica, Pl. XVII., Fig. 6, is found burrowing in floating 
blubber, off the Cape. (Forbes.) 2 living species. 
? Mytilimeria (Nuttallii), Conrad. Shell irregularly oval, thin, 
edentulous, gaping posteriorly ; umbones sub-spiral; ligament 
short, semi-internal. Distribution, California; animal gre- 
garious, forming a nest. 
Modiolopsis (mytiloides), Hall, 1847 ( = Cypricardites, part, 
Conrad. Lyonsia, part, D’Orb.). Shell like modiola, thin and 
smooth, front end somewhat lobed; anterior adductor scar 
* The outer shelllayer has a tubular structure; the tubes are excessively minute, 
celdom branching, oblique and parallel. Carpenter.) 
