DAVIDSON — ON BRITISH CARBONIFEROUS BRACHIOPODA. 
51 
OBSERVATIONS. 
Terebratula. — Four species have 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. Gillincjensis 
and T. vesicularis fi^om Martin's shell. T. vircjoides 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 TerebratuloD 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.fimhriata 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 Sinrigera. — In external shape the species of this genus 
approach more to Terehratula than to any other, and therefore in a 
good or natural aiTangement should preceed Spirifer. Of Athyris, 
eight species have been provisionally retained from among the many 
synonyms, while the value oiA. glohularis 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 ^vith A. squamigera (de Koninck) be correct. 
Of Betzia 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 t^\'ice 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. 
