ASIPHONIDA. 



477 



notch under the anterior ear, and the hinge has one or two 

 cardinal teeth, sometimes with an elongated posterior tooth. 

 How far many of the Palaeozoic shells which have been 

 referred to Avicula can be regarded as truly referable to this 

 genus is doubtful ; but numerous species have been quoted 

 from the Silurian upwards, and undoubted representatives 

 of this type are abundant in the Secondary and Tertiary 

 rocks. Aucella, of the Permian and Secondary deposits, 

 resembles Avicula, but the anterior ear of the left valve is 

 obsolete. In the genus Monotis (fig. 334 D), of the Permian 

 and Triassic rocks, the shell is like that of Avicida in 

 general form ; but the anterior side is short and rounded, 

 and the posterior is slightly eared. The curious genus Posi- 

 donomya fig. 334, F), ranging from the Silurian to the top of 

 the Palaeozoic rocks, comprises singular, thin, concentrically- 



Fig. 334. Types of Aviculidee. A, Avicula Cottaldina Cretaceous; B, Avicula contorta 

 Upper Trias ; c, Vulsella falcata Eocene ; D, Monotis speluncaria Permian ; E, Interior of 

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

 BecJwi Devonian. 



striated shells, with a straight hinge-line, and a general re- 

 semblance in shape to the valves of Estheria. The hinge is 

 toothless, and no ears are developed. Closely allied to the 

 preceding, and to one another, are the genera Halobia and 

 Daonella (fig. 335), of the Triassic period, in both of which 

 are comprised flat, equivalve, radiately - striated shells, 



