422 MANUAL OF THE MOLLTJSCA. 



Distribution, 70 species, chiefly tropical; M. modiolus, Arctic 

 seas — Britain. 



Fossil, 1 50 species. Silurian ? Lias — . United States, 

 Europe, Thibet, South India. 



Sub-genera. Lithodomus, Cuv. M. lithopliaga, PI. XVII., 

 Fig. 7. Shell cylindrical, inflated in front, wedge-shaped behind; 

 epidermis thick and dark; interior nacreous.* Distribution, 

 40 species. West Indies — New Zealand. Fossil, 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 Lithodomi 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. (Lyell's " Principles of Geology.") 



Crenella, Brown. C. discors, PL XVIL, Fig. 8. (Lanistes, 

 Sw. Modiolaria, Beck.) Shell short and tumid, partly smooth, 

 and partly ornamented with radiating striae ; 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. 31. 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, PI. XVIL, Fig. 6, is found burrowing in floating 

 blubber, ofi'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 shell-layer has a tubular stnicture ; the tubes are excessivelj' minute, 

 seldom branching, oblique and parallel. 'Cai-penter. ) 



