1 86 INVERTEBRATE PALAEONTOLOGY 



Rhynchonellids in Mesozoic times. Epithyris^ Glosso- 

 thyris and Musculus showed the typical smooth surface 

 with undulate margin (PL xiii. fig. 8), while Dictyothyris 

 in the Great Oolite, and Terebratulina in the Cretaceous, 

 exhibited features of surface-ornament unusual in the 

 group. Many Chalk forms (e.g. " Terebratula " earned] 

 showed degeneration of the pedicle without any trace 

 of accessory means of adhesion. The Terebratellids, 

 which are the predominant group of recent Brachiopods, 

 appeared in the Lias with the small Megathyris-stnes 

 that still survives; while Coenothyris^ which retained 

 many Terebratulid features, is known from the Trias. 

 Smooth, elongate types (e.g. Aulacothyris, Microthyris 

 and Zeilleria (PI. xiii. fig. 9)) are not uncommon in the 

 Lower Oolites ; and small, short, often superficially 

 ornate genera (Kingena, Trigonosemus y Magas and Tere- 

 bratelld} are more characteristic of the Cretaceous. 



The Spiriferacea were represented by two families 

 which ranged through the Trias into the Lias, and there 

 suffered extinction. The relatively simple and persistent 

 Suessiidae have a familiar Liassic member in Spiriferina 

 (PI. xiii. fig. 10) ; Triassic relatives of this beautiful 

 form were often cemented. The late group Athyridae 

 similarly survived, attaining remarkable complexity (of 

 a gerontic type) in the Alpine Trias. The double spiralia 

 of such types as Diplospirella or Koninckina give striking 

 testimony to the capacities of over-specialization. 



(H) MOLLUSCA 



The disappearance or decline of the most prolific 

 stocks of Brachiopods at the close of the Palaeozoic 

 era left the Mollusca in a predominant position among 

 shell-bearing animals. But the abundance of Mesozoic 

 Molluscs is not merely comparative it is absolute. 



