64 PALAEONTOLOGY 



opisthodetic ligament : only in genera in which the liga- 

 ment becomes shorter do posterior-laterals appear, to 

 strengthen the hinder region, which no longer has a 

 ligamentary connexion. The shell is isomyarian and 

 sinu-palliate, there being a well-marked pallial sinus 

 caused by the large size of the retractor muscles of the 

 long siphons. The valve-margins are sharp and not 

 crenulate. 



Thus Meretrix is heterodont and sinu-palliate, and there- 

 fore representative of what is regarded as the highest 

 type of lamellibranch certainly one of the most modern 

 types. The genus has a world-wide distribution to-day, 

 and species attributed to it are known as far back as the 

 Jurassic. 



7. Pseudomonotis echinata(Fig. 19) is the best pre- 

 served lamellibranch found in the Cornbrash. It is small 

 (length about 14 mm., height 16 mm., thickness 7 mm.), 

 rounded, and strongly inequi valve : the left valve being 

 convex, with prominent umbo, and ornamented with 

 spiny radial ribs ; the right valve being nearly flat, with 

 inconspicuous umbo, and nearly smooth, with only slight 

 radial striae. The outline of each valve is somewhat 

 gibbous oval with the axis curved in a posterior direc- 

 tion ventrally ; but as the outline is traced round to the 

 umbo posteriorly it is seen to be deflected out, giving 

 rise to a flat triangular projection which has the effect 

 of lengthening the hinge-line. Such projections are 

 termed ears (or wings), and a shell possessing them is said 

 to be auriculate (or alate). The posterior ears of Pseudo- 

 monotis are fairly large> the left anterior ear is barely 



