266 MANUAL OF THE MOLLUSCA. 



16 sp. Bath oolite . Europe, TL S. The " date-shell" bores into corals, 

 shells, and the hardest limestone rocks (fig. 25, p. 42) ; its burrows are 

 shaped like the shell, and do not admit of free rotatory motion. The animal, 

 which is eaten in the Medit, is like a common mussel ; in L. patagonicus the 

 siphons are produced. Like other burrowing shellfish, they are luminous. 

 Perforations of LWwdomi 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. (Ly ell's Principles of Geology.) 



Crenella, Brown. C. discors, PL XVII. fig. 8. (Lanistes, Sw. Modio- 

 laria, Beck.) Shell short and tumid, partly smooth, and partly orna- 

 mented with radiating striae ; hinge-margin crenulated behind the ligament ; 

 interior brilliantly nacreous. Animal with the anal tube and branchial 

 margins prominent. Distr. Temperate and arctic seas; Nova Zembla, 

 Ochotsk, Brit. New Zealand. Low-water 40 fms. Spinning a nest, or 

 hiding amongst the roots of sea-weed and corallines. M. marmorata, 

 Forbes, burrows in the test of Ascidia. Fossil, U. Green-sand , Europe. 



Modiolarca (trapezina) Gray ; Falkland Ids. Kerguelen, attached to 

 floating sea- weed ; mantle-lobes united, pedal opening 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?) 



? Mytilimeria (Nuttallii) Conrad. Shell irregularly oval, thin, edentulous, 

 gaping posteriorly; umbones sub-spiral; ligament short, semi -internal. 

 Distr. California ; animal gregarious, forming a nest. 



Modiolopsis (mytiloides) Hall, 1847 (= Cypricardites, part, Conrad. 

 Lyonsia, part, D'Orb.) Shell like modiola, thin and smooth, front end some- 

 what lobed ; anterior adductor scar large and oval. Fossil, Silurian, U. S. 

 Europe. 



? Orthonotus (pholadis) Courad. L. Silurian, New York. Shell elon- 

 gated, margins parallel, umboues anterior, back plaited.* 



DRETSSENA, Van Beneden. 



Etijm. Dedicated to Dreyssen, a Belgian physician. 



Syn. Mytilomya, Cantr. Congeria, Partsch. Tichogonia, Rossm. 



Type, D. polymorpha, PL XVII. fig. 9. (Mytilus Volga, Chemn.) 



Shell like Mytilus., without its pearly lining ; inner layer composed of 

 large prismatic cells; umbones terminal; valves obtusely keeled; right valve 

 with a slight byssal sinus ; anterior adductor supported on a shelf within the 

 beak ; pedal impression single, posterior. 



* Hall and Salter employ the name Orthonotus for such shells as Solen constrictus, 

 Sandb, Devonian, Germany ; Sanguinolites anguliferus, M'Coy, U. Silurian, Kendal; 

 and Solenopsis minor, M'Coy, Carb. limestone, Ireland. M. D'Orbigny has mistaken 

 the plaits for teeth, and placed the genus with Nucula. The recent M. plicata, 

 Lam. from Nicobar Ids. has the same long straight back and plaited dorsal region. 



