204 INVERTEBRATE PALAEONTOLOGY 



by Nucula (which is often common in the Lower 

 Tertiaries of the Hampshire Basin, PL xv. fig. 4), 

 Cucullaea (chiefly in the Woolwich beds) and Glycimeris 

 (PI. iii. fig. 4 and PL xv. fig. 5). The last genus, so 

 abundant on recent beaches, is found in the Eocene, 

 but is especially characteristic of the Pliocene. Among 

 Schizodonts, great reduction on Mesozoic numbers has 

 to be noted. Pinna is one of the few surviving types 

 of the Pteriacea; it is not uncommon in the London 

 Clay. The Inoceramus-senes has disappeared, but Perna 

 still lingers, and even Gervillia has been found in the 

 Eocene. The Ostreidae are the only Schizodont family 

 to maintain supremacy, and Ostrea itself (PL iii. fig. 5), 

 common in all brackish-water facies of the Tertiary, 

 shows few signs of decline at the present day. Unio 

 occurs in some profusion in Oligocene clays; but 

 Trigonia is always scarce in the Cainozoic, and is 

 represented by few living species. The Isodonts retain 

 some measure of their Mesozoic importance. Pecten 

 (PL xv. fig. 6), Spondylus and Anomia are the pre- 

 dominant types. The first is especially abundant in 

 the Pliocene. Many recent species of Spondylus show 

 complete fixation in Oyster-fashion apparently indi- 

 cative of phylogerontic specialization akin to that 

 prevalent in many Brachiopod lineages. Modiolus and 

 Mytilus represent, the Dysodonts, but the latter is less 

 abundant in the fossil state than its modern exuberance 

 would suggest. 



Anomalodesmacea are distinctly less prominent than 

 in the Mesozoic. Pholadomya still persists, and is 

 common in the London Clay. Thracia^ a genus that 

 made its appearance in the Trias, is best known in 

 Britain from the Pliocene. 



Teleodesmacea are developed in overwhelming 

 numbers. The Diogenodonts, which were the only 



