324 SPIRIFERID^. 



detach, leaving cavities ; spiral arms have been observed in all 

 the species. 



MERiSTELLA, Hall, I860. (Pcntagonia, Cozzens ? 1840. Gon- 

 iocoelia, Hall, 1861.) Shell oval, ovoid, orbicular or transverse. 

 Valves unequally convex, with or without a median fold and 

 sinus ; beak apparently imperforate, incurved ; area none. Sur- 

 face smooth or concentrically striated. Dorsal valve with a 

 longitudinal septum ; upper part of the ventral valve with a 

 deep subtriangular muscular impression which unites with the 

 rostral cavity. The species of this group are Meristae without 

 the peculiar appendage of the ventral valve. 1*1 sp. Silurian — 

 Devonian; Europe, N.Am. M tumida, Dalm. (cxxxviii, 20). 



MERiSTiNA, Hall, 1867. Spirals of more simple character than 

 in the typical group. M. nitida, Hall. Upper Silurian. 



WHiTFiP'LDiA, Davidson, 1881. End of loop bifurcated. Sil. ; 

 Europe, America. M. tumida, Dalm. (cxxxvii, 14). 



Bifida, Davidson, 1882. 



Disl7\ — 2 sp. Devonian ; Europe. B. lepida, Groldfuss 

 (cxxxvii, 13). 



Resembles Whitfieldia in the shape and position of the spirals 

 and in the attachments to the hinge-plate, only the spirals of 

 Bifida are slightly depressed or flattened on their dorsal side ; 

 there are usually four coils in each spiral ; the loop is like that 

 in Meristina, with the exception that it is placed nearer to the 

 attachments to the hinge-plate, and that at the point where the 

 two lamellae composing the loop join there is a short bifurcation 

 directed upwards, as in Whitfieldia. 



Retzta, King, 1850. 



Etym. — Dedicated to the distinguished Swedish naturalist, 

 Retzius. 



Syn. — Trigeria, Bayle. 



Distr. — Fossil, about 50 sp. Silurian — Trias ; So. America, 

 United States, Europe. R. trigonella, '^chloih.. (cxxxviii, 21-23). 



Shell punctate, Terebratula-shaped ; beak truncated by a round 

 foramen, rendered complete by a distinct deltidium ; hinge-area 

 small, triangular, sharply defined; interior with diverging shelly 

 spires. 



Professor King first pointed out the existence of calcareous 

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

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

 In form they resemble Terebratulina, Eudesia, and Lyra. 



trematospira, Hall, 1859. {Etym. — Trema, a foramen, and 

 spira.) Shell transverse, elliptical, or subrhomboidal, furnished 

 with internal spires (arranged as in Spirifer); hinge-line shorter 

 than the width of the shell. Valves ai'ticulated by teeth and 

 sockets; beak of ventral valve produced or incurved and trun- 



