214 dr. f. J. stoeth o\ [vol. lxxvi, 



Mutations of Punctospirifer scahrieosta. 



P. scahrieosta is typically developed in the Upper Avonian of 

 the North of England. It is found in the Productus-corrugato- 

 hemisphericus Zone of Westmorland, and in beds of Dibvno- 

 phyllwm age in Northumberland. The forms characteristic of 

 those two horizons possess certain distinctive features. The 

 differences between them are not sufficient to warrant specific 

 separation ; but the later type is undoubtedly a time-variant of 

 that which occurs at the lower horizon, and they may be con- 

 veniently considered as mutations of one species. 



(1) P. SCABRICOSTA, mut. ashfellexsis. (See text-fig. 5o, 



p. 211.) 



Example. — The holotype of P. scahrieosta. 



Horizon. — Productus-corruf/ato-liemispliericiis Zone (S) of 

 the Kirkby-Stephen and Bavenstonedale districts, where it is 

 found in the Ashfell Sandstone in the lower part of that zone, and 

 in the ' Bryozoa-Band ' at the top of the zone (S.,). 



(2) P. scabeicosta, mut. bedesdalexsis. (See text-fig. 5 f r 



p. 211; PL XIII, fig. 6.) 



Examples. — Specimens described by King, Davidson, and 

 others, as Spirifera laminosa} 



Horizon. — Bernician (D |5 Bedesdale Beds) of Redesdale, etc. 

 ( Northumberland). 



These two mutations differ in external features. The mut. 

 ashfellensis approximates most closely to the specific characters 

 already given ; mut. redesdalensis differs from it in the following- 

 respects : — 



(1) The area is a little less elevated, its height being about a quarter of the 

 width, instead of two-fifths, as in mut. ashfellensis. 



(2) The fold in the brachial valve is less elevated, and the sinus in the 

 pedicle-valve shallower. 



(3) The costae are rather more numerous and narrower, there being about 

 nine or ten on each lateral slope, instead of only seven or eight, as in the earlier 

 mutation. 



(c) Carboniferous Species of Spiriferina. 



Forms still retained in Spiriferina occur throughout the Car- 

 boniferous Limestone, from the Cleistopora Zone to the Dibuno- 

 jjJu/llum Zone; they reach their acme in the Upper Dibtmophyllwm 

 Zone, where they include Spiriferina insculpta (Phillips), Sp>. octo- 

 plicata (J. de C. Sowerby), and Sp. ferplicata, sp. nov., species 

 which differ in the number and intensity of their plications. 



Early mutations of the mature forms occur at lower horizons, 

 but they are not common, and have not hitherto been satisfactorily 

 classified. The general tendency has been to refer to them all as 



1 King [17] ; Davidson [4] p. 37. 



