17s 



DR. F. J. NORTH ON 



[vol. lxxvi. 



It will be seen that, apart from the last-mentioned characters 

 (6), the only variable features are, with the exception of the costs, 

 confined to the pedicle-valve, and there do not appear to be any 

 other characters by which specimens showing the brachial valve 

 only can be specifically determined. 



Fig. 3. — The profiles of Syringothyris and Spirifer distans (all 

 half of the natural size). The broken lines represent the 

 floor of the sinus in the pedicle-valve. 



a = Spirifer distans, normal type. 



h = Sp. distans, var. bicarinata. 



c = Syringothyris principalis (K). 



d = S. cuspidata, mut. cyrtorhyncha (Z). 



e = 8. cuspidata, mut. exoleta (C), from normal deposits. 



f = S. cuspidata, mut. exoleta (C), Waulsortian ' knoll '-type. 



g = S. cuspidata, mut. cuspidata (D). 



h = $. elongata (C), Waulsortian ' knoll '-type. 



j = $. elongata (D). 



(e) General Account of Syringothyris in Great Britain. 



Owing to the ease and certainty with which specimens of 

 Syringothyris may be generically determined, on account of the 

 absence of plications upon the fold and the sinus, the evolution 

 of the genus can be studied without the complications which one 

 encounters when dealing with vaguely-defined and obviously-poly- 

 phyletic ' genera ' like Spirifer. 



