MYTILACEA. 



703 



the anterior ear ; the posterior adductor impression is large and sub- 

 central. The shell is inequivalve or subequivalve, oblique, with a 

 straight hinge-line, and furnished on one or both sides with wing- 

 like expansions or ears. Under the anterior ear of the right valve 

 is a notch or aperture for the transmission of the byssus. The 

 family of the Aviculidce is a very large one, and has a most extensive 

 development in past time, beginning under various types in the 

 Ordovician rocks, and being continued thereafter to the present 

 day. The principal forms included in this family may be briefly 

 considered under the following groups : — 



Fig. 568. — Types of Aviculidce. a, Avicula Cottaldi?ia — Cretaceous; b, Avicula contorta — 

 Upper Trias; c, Vulsella falcata — Eocene; d, Pseudomonotis speluncaria — Permian; e, In- 

 terior of the shell of the same, showing the adductor impression and pallial line ; F, Posidonomya 

 Becheri — Lower Carboniferous. 



Firstly, we have the great group of shells, of which Avicula itself 

 (fig. 568, a and b) is the type. In this genus the shell is oblique 

 and inequivalve, the right valve being smaller and less convex than 

 the left. The right valve has a byssal notch under the anterior ear, 

 and the hinge has one or two cardinal teeth, sometimes with an 

 elongated posterior tooth. The ligament is partly external, partly 

 contained internally in a deep groove. The species of Avicula 

 range from the Ordovician period to the present day, numerous 

 forms of this genus occurring in the Secondary and Tertiary 

 rocks. 



More or less closely allied to Avicula are the Palaeozoic genera Leiop- 

 teria (Devonian and Carboniferous), Monopteria (Carboniferous), Pteron- 

 ites (Devonian and Carboniferous), Limoptera (Devonian), and Eopteria 

 (Ordovician). Also related to Avicula is the genus Pseudomonotis {Eumi- 

 crotis, Meek), in which the shell is but slightly oblique, and is inequivalve, 



