BRACHIOPODA. 177 



Genus RHYNCHONELLA, Fischer de Waldheim. 1809. 



The number of palaeozoic species which are currently referred to this genus, 

 and consequently regarded as congeneric with the Russian upper Jurassic 

 R. loxia, Fischer, the type-species, is very great. To the most conservative 

 student such an assemblage, presenting every variety of external configura- 

 tion, must seem more like a hap-hazard and conventional association than a 

 natural group. But we are, nevertheless, here confronted by the fact that 

 leatures of internal structure, upon the variations of which we are wont to base 

 taxonomy, are most persistent. The crura, hinge apparatus and deltidial 

 structure of R. loxia are features which were attained and became fixed in the 

 Silurian period; the extreme pyramidal contour of that species, its smooth 

 surface with few and faint marginal plications, is not, however, except in rare 

 instances, reproduced among the palaeozoic forms. What is thus true of the 

 predecessors of R. loxia is also, to a large degree at least, true of its living 

 descendants. 



From a careful study of the structure of the ancient Rhynchonellas it has 

 become apparent that slight variations from the type of interior possessed by 

 R. loxia are frequently of marked continuance, and we must, therefore, be pre- 

 pared for closer discriminations in this great group of species th.an have else- 

 where been necessary or advisable, and to emphasize such of these deviations 

 from this stable line of development, as are justified by their persistence and 

 the convenience of classification. 



The earlier names introduced among this group of fossils, such as Cyclothyris, 

 McCoy, Hypothyris and Epithyris, Phillips, were based upon the relations of 

 the foramen to the deltidium. It has now become evident that these varying 

 relations are essentially developmental phases. A triangular pedicle-aperture 

 is an immature condition ; it may continue as such even to maturity, or through- 

 out the existence of the individual; it may become closed by normal growth 

 of the deltidial plates, which remain discrete or become united, at first enclos- 

 ing, and perhaps finally obliterating, a subapical foramen; in mature and senile 

 conditions, the aperture if extant, may, by resorption of the shell, encroach 



