170 de. r. j. north on [vol. lxxvi, 



always regarded S. striata as the type-species of Spirifer 1 : and 

 since it is now known that cuspidata does not belong to that 

 genus at all, there are no reasonable grounds for disputing the claim, 

 of the former species to be regarded as the genotype of Spirifer? 



(t/) The Subdivision of the Genus. 



The species which have been referred to Syringothyris, and the 

 specific names which have been applied to various forms occurring 

 in this country are as follows : — 



1. Spirifer distans Sower by. 



2. (Cyrtia) laminosa M ; Coy. 



3. {Anomites) subconicus Martin. 



4. {Anomites) cuspidatus Martin. 



5. (Spirifer) carteri Hall. 



6. Syringothyris typa Winchell. 



7. Syringothyris texta Hall. 



Of these, the first three are shown in this paper to belong to 

 other genera, while the last three are the names of American 

 species to which certain shells from the Lower Avonian of the 

 South- Western Province have been doubtfully referred. The use 

 of these names is, however, undesirable, partly because of the 

 absence of facilities for comparison with the type-specimens, and 

 partly because American palaeontologists are themselves not in 

 agreement as to their precise significance (see p. 174). Therefore, 

 so far as British specimens are concerned, the only valid specific 

 name already proposed is cuspidal a. 



If isolated specimens from various horizons and different localities 

 were considered separately, it would be comparatively easy to 

 divide the genus into many apparently well-defined species ; but, 

 as is often the case, the greater the number of specimens examined 

 the more difficult specific differentiation becomes. The changes 

 dependent upon time alone are small and progressive ; but, in 

 deposits such as the Waulsortian ' knoll '-limestones, where the 

 conditions of deposition were somewhat abnormal, the shells at any 

 one horizon exhibit considerable variation. 



The characters in Avhich variation occurs are as follows : — 



(1) The height of the area. — Generally speaking, the area increases 

 in height from lower to higher horizons ; but, as with the other characters, the 

 greatest amount of variation is due to the existence of a special environment, 

 and not to considerations of time (see fig. 3, p. 178). 



1 Davidson [3] p. 81. 



2 Since these lines were written, Dr. J. Allan Thomson has cited the case 

 of Syringothyris cuspidata and Spirifer as an example of the necessity for a 

 list of nomina conservanda approved by the International Zoological Congress, 

 so that in this and similar cases, it should not be necessary by the strict 

 application of the law of priority to displace a name which, like Spirifer, has 

 been in general use, in a well-defined sense, for a long period. See ' Brachiopod 

 Nomenclature : Spirifer & Syringothyris ' Geol. Mag. 1919, pp. 371-74 ; also 

 S. S. Buckman, Geol." Mag. 1920, pp. 18-20. 



