THE TRIASSIC PERIOD. 211 
become apparently extinct. The same is true of many of the 
ancient types of Srachiopods, and conspicuously so of the 
great family of the Productide, which played such an important 
part in the seas of the Carboniferous and Permian periods. 
Bivalves (Lamellibranchiata) and Univalves (Gasteropoda) 
are well represented in the marine beds of the Trias, and 
some of the former are particularly characteristic either of the 
formation as a whole or of minor subdivisions of it. A few of 
these characteristic species are figured in the accompanying 
illustration (fig. 144). Bivalve shells of the genera Daonella 
(fig. 144, a) and Halobia (Monotis) are very abundant, and are 
e4,)} 
Ly Mp 
wy) 
Fig. 144.- Triassic Lamellibranchs. a, Daonella (Halobia) Lommelli; b, Pecten 
Valontensis; c, Myophoria lineata; ds Cardium Rheticum; e, Avicula contorta; 
J; Avicula socialis. 
found in the Triassic strata of almost all regions. These 
groups belong to the family of the Pearl-oysters (Aviculide), 
and are singular from the striking resemblance borne by some 
of their included forms to the Strophomene amongst the Lamp- 
shells, though, of course, no real relation exists between the 
two. The little Pearl-oyster, Avecula soctalis (fig. 144, f), is 
found throughout the greater part of the Triassic series, and is 
especially abundant in the Muschelkalk. The genus JZjo- 
phoria (fig. 144, c), belonging to the Z7igoniade, and related 
therefore to the Permian Schizodus, is characteristically Trias- 
sic, many species of the genus being known in deposits of this 
age. Lastly, the so-called “ Rhetic” or “ Kossen” beds are 
