214 BRITISH CARBONIFEROUS BRACHIOPODA. 



each of the above-named forms, has shown so many intermediate shapes, that it must 

 remain a question whether the last three are in reality more than varieties or simple 

 modifications of the first in shape. 



At page 12 I mentioned that Mr. De Verneuil, and several other experienced 

 palaeontologists, were of opinion that T. sacculus was only a variety of T. hastata ; I also 

 stated (p. 17) that T. Gillinr/ensis had been supposed by some observers to be a variation 

 in form or the young of T. hastata, and while describing T. vesicularis (p. 16), I did not 

 omit to remind the reader that De Koninck's species was extremely variable both in shape 

 and character, so much so, that, to my eyes, certain examples did appear undistinguishable 

 from others of Martin's T. sacculus, and to which, Prof. De Koninck admits his shell 

 to be very closely related. No one will, therefore, feel much surprised w T hen I affirm 

 that it is impossible to determine whether very many intermediate shapes or specimens 

 should be referred more to one than another of the four above-named so-termed species, 

 and that there is absolutely no line of demarcation between any of the four forms above 

 recorded. 



It would therefore not be surprising, if all the British Carboniferous Terebratulse hitherto 

 discovered, were to prove mere modifications of a single very variable species, capable of 

 assuming different shapes (influenced no doubt from local circumstances), and not pre- 

 senting a greater extent of modification than what is common to many other species. For 

 example, is not Terebratula plicata and T.fimbriata entirely smooth up to a certain age, 

 and indeed often so to an advanced stage, when they afterwards suddenly or by degrees 

 become more or less regularly plicated during the remaining period of their growth ? And 

 many other examples could be given of still greater modifications. 



T. hastata is the largest British Carboniferous Terebratula with which we are at present 

 acquainted, some specimens having attained twenty-six and a half lines in length, nineteen 

 and a half in width, and thirteen in depth (PI. XLIX, fig. 11), which is certainly the 

 full-grown condition of the species, but which, even under the most favorable conditions, 

 was exceedingly variable, as may be seen by casting a glance at the numerous examples 

 represented in PL I and XLIX of the present Monograph. 



T. sacculus, in its typical shape, appears to be a thickened dwarfed condition of 

 Sowerby's species, and although it has been urged that Martin's shell was never coloured 

 (so far as known), while T. hastata was ornamented with purple-colour bands, it must be also 

 remembered that the number of specimens discovered which have shown these remains of 

 colour have been few ; and that we are by no means certain that T. sacculus may not have 

 been similarly ornamented. It is likewise not correct to say, that T. hastata always 

 possessed sharp edges, for if we examine a large series of specimens we will soon perceive 

 among them many as thick edged as any hitherto discovered of the so-termed T. sacculus. 



If the reader will refer to PI. VI of my Cretaceous Monograph, he will find 

 therein many modifications of T. b/plicata carefully represented, and will, I am sure* 

 perceive as great a difference between certain specimens of this cretaceous shell as any he 



