CONCHIFERA. 
of strize, which radiate from the beaks, leaving the middle 
portion smooth, umbones incurved, hinge SSO or cre- 
nulated, hinge-plate finely notched. 
Distribution, Temperate and Arctic seas. The four British 
species occur fossilised in the Red and Coralline Crags and 
newer Tertiaries. Several species in the Upper Triassic and 
Jurassic formations, referred to Modiola, appear to belong here. 
CRENELLA, Brown (see p. 422). 
Etymology, diminutive of crena, a notch. 
Example, Mytilus decussatus, Montagu. 
Animal with the mantle open in front, and folded behind 
into a sessile excurrent tube; foot cylindrical, the free end 
being disk-like and issuing out of a sheath. 
Shell oval or rhomboidal, nacreous, cancellated; umbones 
straight, ligament small, hinge of each valve furnished with an 
upright tooth, which is crenulated, as well as the hinge plate. 
The animal does not spin a thick byssus, ike Modiolaria, 
but secretes only a single thread for attachment, and by means 
of which it holds itself suspended in the water. 
Distribution, 5 species. Low-water mark to 150 fathoms. 
Norway, Iceland, Greenland, New England, Britain, France. 
C. rhombea occurs in a fossil state in the Coralline Crag, 
Sutton. 
b 
PrASINA, Deshayes, 1863. 
Type, P. Borbonica, Desh. Isle of Bourbon. 
Shell oblong thick cordiform, valves closed, margins entire 
inequilateral; lunule deep circular, projecting into the interior 
of the right valve, left valve in the same place furnished with 
dentiform tubercles; hinge line simple, arched; ligament 
external, narrow; muscular scars two, unequal, sub-central. 
ANTHRACOPTERA, Salter, 1863. 
Etymology, anthrax, coal, and pteron, a wing. 
Example, A. Carbonaria, Dawson, sp. 
This genus includes the so-called I, yaline, but they have 
not the thick hinge-plate of the shells of that genus, and 
species which have been described by Ludwig as belonging to 
Dreissena. The form of the shell is triangular. 
Fossil, 7 species of marine origin. Coal Measures. Great 
Britain, Nova Scotia, Westphalia, 
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