MYTILACEA. 



707 



nella (fig. 574) and Halobia, in which are comprised flat, equi- 

 valve, radiately-striated shells, more or less markedly inequilateral 

 in shape, with a straight hinge-line, without a ligamental pit or teeth, 

 and with no ears. Also belong- 

 ing to this group is the remark- 

 able genus Posidonomya ( = Po- 

 sidom'a), in which the shell (fig. 

 568, f) is thin, concentrically 

 striated, with a straight hinge- 

 line, destitute of hinge-teeth, 

 and without developed ears. 

 The species of Posidonomya 

 ran^e from the Silurian to the 

 Jurassic rocks, and have a gen- 

 eral resemblance in shape to specimens of Estheria. The species 

 of this genus commonly occur gregariously, and particular species 

 are commonly characteristic of particular horizons. 



The important Palaeozoic genus Pterinea is the type of another 

 group of the AviculidcE, in which the shell (figs. 575, 576) is equi- 



Daonella (Halobia) Lommelli. Trias. 



Fig. 575. — a, Pterinea subfalcata, 

 Silurian. (After M'Coy.) B, Interior 

 of the left valve of Pterinea Icevis, De- 

 vonian. (After Zittel.) 



Fig. 576. — Pterinea (AtucuIo) demissa. 

 Ordovician rocks (Hudson River group) of 



Nurth America. 



valve, inequilateral, and winged, with a byssal notch below the 

 anterior ear. The hinge has two or more cardinal teeth, with 

 oblique lateral teeth, and there is a large, longitudinally-striated 

 ligamental area for the reception of the internal ligament. The 



