Yol. 62.] IN THE CARBONIFEROUS ROCKS AT RUSH. 305 



VII. Notes on the Genera, and Species cited in the 

 Patjnal Lists. [A. V.] 



If Pbe a generic name and x a specific name, Pcf. x implies that, 

 of all the named species of the genus P, x is that one which 

 presents the closest resemblance to the form denoted by P cf. x. 



By the use of this notation, it is further implied that there is 

 no reason for believing that Px and P cf. x are links or 

 appendages of the same chain of evolution (that is, Px and P cf. x 

 are not members of the same gens). 



P aff. x is any one of the links or appendages of the chain of 

 evolution which contains the named species Px (that is, Px 

 and P air*, x are members of the same gens). 

 Px cvgt. Qy implies convergence or assimilation between two 



contemporaneous species belonging to distinct genera. 

 Two cases of convergence are of common occurrence : — 



(1) A general structural convergence which affects, in greater or less degree, 



all the genera of similar organisms living at the same time (for example, 

 the general adoption of a Clisiophyllidan structure by the D 2 corals). 

 This phenomenon may be termed a time-trait. 



(2) A local convergence (or mimicry) by which the external form of two 



species belonging to distinct genera, but living in association, tends to 

 become identical (for example, Martinia glabra and Athyris glabristria 

 in the Curkeen Limestone). 



A compound specific term, such as Pyx, indicates any form 

 which varies from the type-species Px in the direction of the 

 type-species Py ; hence Pyx indicates an entire segment of a 

 chain of evolution, extending from Px to Py, whereas Px and Py 

 represent points only. 



The employment of compound terms is, in many cases, more 

 appropriate than the creation of new species when we are dealing 

 with large divisions of time, such as are implied in zones ; and 

 especially is this the case when the whole of the segment represented 

 by Pyx lies entirely within the zone. 



BBACHIOPODS. 

 Semireticulate and scabriculate Producti. 



Since I am now engaged upon the detailed study of this group, 

 it will be sufficient here merely to schematize the lines of variation 

 which are exhibited by the group in the Bush sequence. 



In the appended scheme (fig. 12, p. 306), an unmodified specific 

 name denotes the type of the original author. 



Pustulose, fimbriate, and punctate Producti. 



The Producti of this group are all ornamented with numerous, 

 regularly-arranged, short, radial, procumbent spine-bases. 



