708 



LAMELLIBRANCHIATA. 



small impression of the anterior adductor is placed below the an- 

 terior ear, and the surface is radiately striated. The species of 

 Pterinea have a wide distribution in the Ordovician, Silurian, De- 

 vonian, and Carboniferous rocks. The Devonian genus Glyfttodes- 



ma is allied to Pterinea, but the liga- 

 ment is external, and the hinge has 

 two strong lateral teeth, together 

 with numerous irregular transverse 

 plications along its margin. 



The genus Ambonychia (fig. 577) 

 is the type of another group of the 

 Aviculidce, and is widely distributed 

 in the Palaeozoic rocks, ranging from 

 the Ordovician to the Carboniferous. 

 In this genus the shell (fig. 577) is 

 equivalve and very inequilateral, the 

 beaks being pointed, and placed at 

 the anterior end of the long straight 

 hinge-line. The ligament is internal, 

 and is lodged in longitudinal grooves 

 running parallel with the cardinal 



Fig. 577. — Ambonychia radiata. 

 Ordovician. 



margi 



There is no anterior ear, 



but a wide byssal aperture is present, 

 while the posterior ear is broad and aliform. The Devonian 

 genera Mytilarca and Gosseletia may be placed in the neighbour- 

 hood of Ambonychia. 



A transition between the Aviculidce and the Mytilidce is effected 



Fig. 578. — Myalina crassa, interior of a broken right valve, showing the hinge. 

 Carboniferous Limestone. (Original.) 



by the remarkable genus Myalina (fig. 578), in which the shell is 

 inequivalve, and mussel-shaped, with terminal beaks, and a flat and 

 thickened cardinal area marked with longitudinal cartilage-grooves, 

 and sometimes with the umbones septate. The genus Myalina is 



