374 



MANUAL OF THE MOLLUSCA. 



Snb-genus ? Merista^ Suess. Ter. scalprutn, RoDmei'. (A. 

 cassidea, Quenst. Sp. plebeia, Ph.) Silurian 

 — Devonian. Europe. Shell impunctate, 

 dental plates {v) and dorsal septum {d) sup- 

 ported by arched plates ("shoe-lifter" 

 processes, of King) which readily detach, 

 leaving cavities (as in Pig. 171) ; spiral arms 



Fig. 171. Merista. \i^yQ been observed in all the species. 



Retzia, King. 



Dedicated to the distinguished Swedish naturalist, Retzius. 



Type^ Ter. Adrieni, Vern. 



Example, E. serpentina, Carb. L., Belgium, Pig. 172. 



Shell punctate, terebratula- shaped ; beak truncated by a 

 round foramen, rendered complete by a distinct deltidium ; 

 hinge-area small, triangular, shar^jly defined; interior with 

 diverging shelly spires. 



Fossil, about 50 species. Silurian — Trias. South America, 

 United States, Europe. 



Professor King first j)ointed out the existence of calcareous 

 spires in several Terehratidce of the older rocks, and others have 

 been discovered by MM. Quenstedt, De Koninck, and Barrande. 

 In form they resemble Terehratulina^ Eiultsia, and Lyra, 



Fig. 17'i. Retzia serpentina, D. A' 



Fig. 173. Uncites gri/jhus. 



Uncites, Defrance. 



Type, V. gryphus, PL XV., Fig. 17. Fig. 173. 



Fossil, Devonian. Europe. 



Shell impunctate ; oval, bi-convex, with a long incurved 

 beak ; foramen apical, closed at an early age ; deltidium large, 

 concave ; spiral processes directed outwards ; no hinge-area. 



The large, concave deltidium of Uncites so much lesembles 

 the channel formed by the dental plates of Fentanurus, that 



