[Footnote 22: The genus _Retefora_ is really a recent one, represented by living forms; and the so-called _Reteporoe_ of the Palæozoic rocks should properly receive another name (_Phyllopora_), as being of a different nature. The name _Retepora_ has been here retained for these old forms simply in accordance with general usage.] [Illustraton: Fig. 144. Triassic Lamellibranchs. a, _Daonella_ (_Halobia_) _Lommelli_; b, _Pecten Valoniensis_; c, _Myophoria lineata_; d. _Cardium Rhoeticum_; e. _Avicula contorta_; f. _Avicula socialis_.] _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 found in the Triassic strata of almost all regions. These groups belong to the family of the Pearl-oysters (_Aviculidoe_), and are singular from the striking resemblance borne by some of their included forms to the _Strophomenoe amongst the Lamp-shells, though, of course, no real relation exists between the two. The little Pearl-oyster, _Avicula socialis_ (fig. 144, f), is found throughout the greater part of the Triassic series, and is especially abundant in the Muschelkalk. The genus _Myophoria_ (fig. 144, c), belonging to the _Trigoniadoe_, and related therefore to the Permian _Schizodus_, is characteristically Triassic, many species of the genus being known in deposits of this age. Lastly, the so-called "Rhætic" or "Kössen" beds are characterised by the occurrence in them of the Scallop, _Pecten Valoniensis_ (fig. 144, b); the small Cockle, _Cardium Rhoeticum_ (fig. 144, d); and the curiously-twisted Pearl-oyster, _Avicula contorta_ (fig. 144, e)--this last Bivalve being so abundant that the strata in question are often spoken of as the "Avicula contorta beds." [Illustration: Fig. 145.--_Ceratites nodosus_, viewed from the side and from behind. Muschelkalk.]