BRACHIOPODA. 
71 
their apices directed toward the lateral margins. The loop has been shown by 
the Rev. Norman Glass to have the following structure; the lateral branches 
approach and unite near the middle of the 
interior cavity, forming a very short stem, 
from the posterior extremity of which is 
given off a pair of arms. These curve down¬ 
ward to the primary lamellsB of the coil and 
returning, meet the lateral branches below 
their point of union; the whole forming a 
scissors-shaped arrangement essentially like 
that of Meristella, differing only in minor 
respects indicated under the discussion of The spirals and loop of ilfemia ftercuiea, 
.1 , Barrande. (After Davidson, from a prepara- 
mat genus. glass.) 
External surface of the valves smooth or with concentric growth-lines. 
Shell-substance fibrous. 
Type, Terebratula herculea, Barrande. Etage E. 
Observations. Merista is a genus rather sparingly represented in species, 
though some of the species, like M. scalprum, Roemer ( — M. pkbeia, Sowerby), 
of the European middle Devonian, are very abundant in individuals. In 
American faunas there are but three forms which may at present be refer¬ 
red to the genus, M. typa. Hall, M. elongata, Hall, a probable variety 
of the former, from the Lower Helderberg fauna at Cumberland, Maryland, 
and a new species, M. Tennesseensis, from a similar horizon in Perry county, 
Tennessee. 
It appears from the description of the genus above given, that the essential 
difference between Merista and Meristella lies in the existence, in the former, 
of the plate termed by King the “shoe-lifter process”; the variations in the 
structure of the loop and hinge-plate being of minor importance. This internal 
plate, free at its inner edge, must have induced some important modification 
in the functions and internal arrangements of the animal. It is evident that 
its upper surface was one of muscular insertion, and whatever may have been 
