324 SPIRIFERIDA. 
detach, leaving cavities; spiral arms have been observed in all 
the species. 
MERISTELLA, Hall, 1860. (Pentagonia, Cozzens ? 1846. Gon- 
ioceelia, 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 Meriste without 
the peculiar appendage of the ventral valve. 17 sp. Silurian— 
Devonian; Europe, N. Am. MZ tumida, Dalm. (exxxviii, 20). 
MERISTINA, Hall, 1867. Spirals of more simple character than 
in the typical group. J. nitida, Hall. Upper Silurian. 
WHITFIFLDIA, Davidson, 1881. End of loop bifurcated. Sil. ; 
Europe, America. MM. tumida, Dalm. (exxxvii, 14). 
Birrpa, Davidson, 1882. 
Distr.—2 sp. Devonian; Europe. 2B. lepida, Goldfuss 
(cxx xvii, 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 lamellze composing the loop join there is a short bifurcation 
directed upwards, as in Whitfieldia. 
Rerzia, 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, Schloth. (exxxviii, 21-23). 
Shell punctate, Terebratula-shaped ; beak truncated by a round 
foramen, rendered complete by a distinct deltidium ; hinge-area 
small, triangular, s sharply defined; interior with dive erging - shelly 
Spires. 
Professor King first pointed out the existence of calcareous 
spires in several Terebratulee 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. (Htym.— 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 articulated by teeth and 
sockets; beak of ventral valve produced or incurved and trur- 
