MESOZOIC FAUNAS 185 



deposit was at one time considered to be an outlying 

 portion of the Pliocene " Coralline Crag " on account 

 of its Polyzoan wealth ; but detailed study of the fauna 

 showed that it marks comparable conditions of sedi- 

 mentation in Lower Cretaceous times. 



(G) BRACHIOPODA 



Save for a few degenerate or over - specialized 

 survivors of Atremata, Neotremata, Protremata and 

 Spiriferacea, the Mesozoic Brachiopod fauna may be 

 summarized by the broad "generic " terms Rhynchonella, 

 Terebratula and Terebratella. Lingula, Orbiculoidea and 

 Crania represented the two first orders with no greater 

 distinction than during the Upper Palaeozoic. Small, 

 usually cemented forms allied to Thecidea were the only 

 Protremata of the Mesozoic, as they are of the present day. 



The Rhynchonellids are abundant throughout the 

 Mesozoic, and may be found at almost all marine 

 horizons of the Liassic, Lower Oolitic and Cretaceous 

 strata of Britain. Most of the later Mesozoic types 

 had a sharp parapet around the pedicle-aperture, and 

 are thence styled Cyclothyris (PL xiii. fig. 7), but a vast 

 series of generic terms are applied to Jurassic forms. 

 There seems a fairly steady tendency towards increasing 

 delicacy of ornament as the group is traced from lower 

 to higher stages. Some Lower Oolitic types, such as 

 Rhynchonella cynocephala, are almost Pugnax-like in 

 form and boldness of plication ; while the Cretaceous 

 fades shown in Cyclothyris latissima or C. plicatilis 

 (PL xiii. fig. 7) is flattened and finely ribbed. One 

 series, illustrated by Acanthothyris, developed a spinose 

 surface suggestive of racial decadence ; the genus named 

 js locally abundant in the Inferior Oolite. 



Terebratulids were almost more prominent than 



