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 arras. These curve down- 

 ward to the primary lamellae 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 

 that genus. 



External surface of the valves smooth or with concentric growth-lines. 

 Shell-substance fibrous. 



Fig. 61. The spirals and loop of Merista herculea, 

 Barrande. (After Davidson, from a prepara- 

 tion by Glass.) 



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 



