CONCHIFERA. 



of striue, which radiate from the beaks, leaving the middle 

 portion smooth, umbones incurved, hinge edentulous or cre- 

 nulated, hinge-plate finely notched. 



Distribution, Temperate and Arctic seas. The four British 

 species occur fossilised in the Eed 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, like 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. 



0. rhombea occurs in a fossil state in the Coralline Crag, 

 Sutton. 



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 Myctlince, 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. 



60 



