BKACHIOPODA 



415 



^^^RiASS II 



^B Family 7. Meristellidae Hall and Clarke. 



Derived Spiriferacea, with the bases of the primary lamellae situated between the 

 spiralia and sharply recurved dorsally at their junction with the crura. Spiralia 

 laterally direded. The jugum has a Single process that may remain simple or may 

 bifurcate ; the bifurcations, however, do not enter between the lamellae of the spiralia^ 

 but recurve and join the jugal process near their origin. Shell usuülly smooth, but 

 sometimes finely hirsute, and the structure impundate. Late Ordovician to 

 Devonian. 



Siibfainily A. Hindellinae Schuchert. 



Primitive Meristellidae in which the jugum has a single process that is usuwUy 

 simple and rarely is sharply recurved terminally. 



Hindella Davidson. Ovate or elongate, sub-equally convex, smooth, 

 meristelloid shells. The V-shaped jugum has a short, acute process. A 

 dorsal median septum present. Late Ordovician ; North America. Sub- 

 genus Greenfieldia Grabau. Has no median dorsal septum. Late Silurian ; 

 North America. 



Whitfieldia Hall and Clarke. Externally sometimes like Hindella, but 

 usually the shells have a fold and sinus, 

 the spirals have more volutions, and the 

 jugal process is longer and curved. 

 Genotype W. nitida H. and C. Silurian 

 and Lower Devonian ; North America 

 and Europe. 



Hyattidina, nom. nov. (Hyattella Hall 

 and Clarke, non Fer. 1821). Similar 

 to Hindella, but compactly sub-penta- 

 hedral, and without the dorsal median 

 septum. Late Ordovician and early England 

 Silurian ; North America. 



Nucleospira Hall (Fig. 629). Sub-circular, biconvex shells with numerous 

 fine, short spinules. Jugum with a long, straight, simple process. Silurian 

 to Lower Carboniferous ; North America and Europe. 



Fig. 629. 

 Nucleospira pisum (Sow.). Silurian ; Wenlock, 

 Interior of dorsal valve, B, Vertical 

 section through both valves, s/g (after Davidson). 



Subfamily B. Meristellinae Waagen. 



Specialised Meristellidae, in which the jugal process bifurcates and may remain 



so or may continue to grow, forming two loops as in the handles of scissors. 



Meristina Hall {Whitfieldia Davidson) (Fig. 630). Biconvex, smooth- 

 shelled. Jugal stem with a short bifurcation. Silurian; North America 

 and Europe. 



Glassina Hall and Clarke. Like Meristina^ but with the bifurcations of 

 the jugum originating directly from its apex. Silurian ; England. 



Meristella Hall. Externally like Merista, but without spondylia. Apex 

 of jugum with two annular processes. Devonian ; North and South America 

 and Europe. 



MeristoSpira Grabau. Like Meristella, but with strong dental lamellae. 

 Hinge-plate perforated by a visceral foramen and the dorsal septum not united 

 with the hinge-plate. Late Silurian ; North America. 



