DAVIDSON — OX BRITISH CARBONIFEROUS BRACHIOPODA. 



51 



OBSERVATIONS. 



Terebratula. — Four species ha^e been provisionally admitted; but 

 as they appear all so closely connected by intermediate or passage 

 shapes, it may still remain a question whether they in reality are 

 more than varieties or modifications in shape of a single species ? It 

 has often been said and thought that T. hastata was no more than 

 an elongated full-grown condition of Martin's T. sacculus, and it is at 

 times hardly possible to distinguish certain examples of T. Gillingensis 

 and T. vesicular is from Martin's shell. T. virgoides has been supposed 

 to be distinct from T. hastata ; but after a lengthened examination of the 

 original specimen figured in the " Synopsis," and another from the 

 same locality (Windmill, in Ireland), I could not make up my mind 

 to separate it from T. hastata, to some specimens of which it bears 

 much resemblance. T. vesicularis is a very variable shell ; for, while 

 some specimens present the deep triundate or triplicated dorsal valve, 

 or frontal margin, in the greater number of individuals this is very 

 slightly marked, and even absent. T. vesicularis was for long believed 

 to be a small shell not exceeding seven lines in length, but some 

 large examples recently discovered at Bowertrapping, in Scotland, 

 have exceeded an inch in length. 



It would, therefore, not be impossible that all the British Carboni- 

 ferous Terebratula? hitherto discovered may, perhaps, belong to a 

 single species, capable of assuming different shapes, and not pre- 

 senting a greater extent of modification than what we find in the T. 

 Australis as well as in many other recent and fossil species. Are not 

 the Jurassic Ter. plicata and T. fimbriate/, entirely smooth up to a cer- 

 tain age, and indeed often so to an advanced age, when they suddenly, 

 or by degrees, becomes more or less regularly or irregulary plicated 

 during the remaining period of their growth? : For the present, how- 

 ever, and until our ideas as to the absolute necessity of enlarging the 

 circle or range of variation to be permitted to a species be admitted 

 and understood, the four species of Terebratula recorded may be 

 provisionally retained. 



Athyris or Spirigera. — In external shape the species of this genus 

 approach more to Terebratula than to any other, and therefore in a 

 good or natural arrangement should preceed Spirifer. Of Athyris, 

 eight species have been provisionally retained from among the many 

 synonyms, while the value of A. globularis and A. squamigera may still 

 require confirmation, for of both these shells the material at my 

 command has been very scanty ; and it is even uncertain whether 

 the identification with A. squamigera (de Koninck) be correct. 



Of Retzia there appears to exist two species, of which _B. radialis is 

 both the less rare and most variable shell; for in some localities it 

 appears to occur as a small race with slender ribs, which in other 

 localities individuals twice the size with stronger ribs are prevalent. 

 Of Betzia ulotrix I am acquainted with but two or three British 

 examples, so that a search for more would be very desirable. 



