MESOZOIC FAUNAS 191 



abundant also in the Gault (Gyrodes}. The fresh-water 

 Vivipara (Paludind) is a local rock-former in the 

 Wealden. Turritella, Cerithium and Aporrhais are 

 especially Cainozoic forms, but they had some abundant 

 species in the Mesozoic, the first and last being particu- 

 larly common in the Selbornian. A peculiar series, the 

 Nerineidae, with complex internal folds, is character- 

 istic of the era, Nerinea being especially abundant 

 in the Great Oolite and Corallian. Rachiglossa and 

 Toxoglossa, which attain remarkable differentiation in 

 modern faunas, appeared first in the Cretaceous, but 

 are rarely found in British strata of that age. 



The small group of Tectibranchiate Euthyneura 

 reached its maximum in the Mesozoic. Actaeonina, Cylin- 

 drites and Tornatellaea are common Jurassic fossils. 

 Pulrnonata are represented but sparingly. 



Nautiloid Cephalopods were restricted to Nautilus and 

 genera with similar form. Nautilus itself is an essen- 

 tially Mesozoic genus (often attaining great profusion), 

 and is the sole modern survivor of its order. Am- 

 monoids reached a dazzling acme in the Trias, but 

 geological conditions make them inaccessible to British 

 collectors at that period. Nevertheless, from the Lias 

 to the Cretaceous they are the most abundant and 

 striking fossils of most horizons. The greatest degree 

 of septal complexity was attained by Pinacoceras in the 

 Trias, while small forms akin to Lobites reached the 

 fullest stage of involution in the same period. But 

 phylogerontic forms, such as Cochloceras and Rhabdoceras> 

 mark the failure of some lineages at the opening of 

 Mesozoic time. The most abundant Ammonites of the 

 Lower Lias are Tragophylloceras (Phylloceratidae), 

 PsiloceraSy Coroniceras and Asteroceras (Aegoceratidae), 

 and Oxynoticeras (? Amaltheidae). Common Middle Lias 

 genera are chiefly Amaltheidae (Amaltheus, PI. xiv. fig. 6). 



