GENERA OF MISSISSIPPIAN LOOP-BEARING 

 BRACHIOPODA 



STUART WELLER 



Introduction 



The correct specific determination of the loop-bearing brachio- 

 pod shells of the Mississippian faunas has always been attended 

 with difficulty. This condition led the writer to undertake a 

 critical study of a large amount of material, in order to determine, 

 if possible, the true criteria for the determination of species. In 

 this study the internal characters of the shell, as well as the exter- 

 nal configuration, have been taken into consideration. 



In the earlier literature all the shells of this type were included 

 in the genus Terebratula, but since the appearance of Hall and 

 Clarke's great work on the Genera of Paleozoic Brachiopoda, 1 it has 

 been the usual custom to refer all of them to the genus Dielasma, 

 although Girty has described one form as a member of the genus 

 Harttina. 2 As a result of the present investigation, however, it 

 has been found that forms which have been commonly included in 

 a single genus, and in some instances, even, forms which have been 

 referred to a single species, in reality represent several perfectly 

 distinct generic types. The method used in the investigation is 

 that which has been formerly used in the study of the Rhyncho- 

 nelloid shells. Specimens have been ground from the posterior 

 extremity and at short intervals the ground surface has been pol- 

 ished and a careful drawing made of the cross-section of the shell 

 parts shown. From such a series of cross-sections of any shell it 

 is easy to recognize the character and position of the internal 

 lamellae, such as the median septum, the hinge-plate, the socket- 

 plates, the bases of the crura, etc. In the course of the study it 

 has been shown that the characters which can be considered as of 



1 Paleontology of New York, Vol. VIII, Parts i and 2. 



2 "Harttina indianensis Girty," Proc. Nat. Mus., Vol. XXXIV, p. 293. 



439 



