704 



LAMELLIERANCHIATA. 



the left valve convex, with a prominent beak. The hinge is nearly or 

 quite toothless, and the anterior ear is small or undeveloped, while a deep 

 byssal notch in front is present (fig. 568, D and E). The earliest forms of 

 Pseudomo7iotis are found in the Devonian rocks, but the genus attains its 

 maximum in the Triassic and Jurassic rocks. A well-known species from 

 the Permian rocks of Europe is P. spehmcaria (fig. 568, d). In the 

 singular Triassic genus Cassia?tella (fig. 570, D and e) the shell is thick 

 and very inequivalve, the right valve being flat or concave, while the left 

 is strongly convex. The hinge-line is straight, with few teeth, the liga- 

 ment being lodged in a triangular groove behind the beaks ; a byssal 

 notch is absent ; and the shell is eared. 



Another, but limited, group is that represented by Vulsella and 

 Malleus, together with some Secondary and Tertiary forms of small 

 importance, characterised by the common feature that the ligament 

 is lodged in a single pit extending from the beak internally. In 

 Vulsella (fig. 568, c) the shell is nearly equivalve and earless, the 

 hinge being toothless, and a byssal notch being absent. The genus 

 dates from the Eocene Tertiary, and still survives. On the other 



Fig. 569. — Perna Mulleti (a), and a portion of its hinge-line (b). Lower Greensand. 



hand, the " Hammer-oysters " {Malleus) are not known in the fossil 

 state, and are distinguished by their very long ears and central 

 beaks. 



A third, much more important, group of the Aviculidce is that 

 represented by Perna and its allies, all of which have a straight 

 hinge-line, crossed by numerous transverse furrows for the lodgment 

 of the ligament. In Perna (or Melina) itself (fig. 569) the shell 

 varies in form, but there is generally a long posterior ear ; there are 

 numerous close-set cartilage-pits, and the right valve has a byssal 



