THE BRITISH FOSSIL BRACHIOPODA. 361 



genus is entirely absent. Dr. Waagen places in the genus Dieiasma, Terebratula sacculus, 

 Martin; and perhaps T. sujla la, Sch\.; T.Jlcus, M'Coy; T. virgoides ,M' 'Coy; T. hastceformis, 

 de Kon. ; T. plica, Kutorga ; T. Gillingensis, Dav. ; T. elongata, Schl., &c. •} and he does 

 not fail to remark, at p. 338 of his work, that " There decidedly exist intermediate forms 

 between all the above-mentioned species or sub-species or whatever we like to call them ; 

 and a perfect transition can be traced from one form to the other. It cannot be doubted, 

 (and the standard of thorough study and excellent power of observations exhibited by all 

 the works of Mr. Davidson is an absolute security for this), that the transitional forms 

 really exist, and that all the fossils mentioned under the above names are all most 

 intimately connected together ; nevertheless, I must retain the opinion that these fossils 

 have all to be considered as distinct species. Any one who has collected Brachiopods 

 in large numbers in the field will have made the observation that in this class of 

 Molluscs, more than in any other classes, transitional forms between different species 

 occur, and that such transitional forms are chiefly numerous in certain groups, for instance, 

 in the biplicate Terebratula, more than in others. That such transitional forms occur 

 between species which are of different geological age is only natural, and depends 

 upon the mode of development of the species ; but also between contemporaneous 

 species transitional forms exist. In this latter case, the distribution of the transitional 

 forms is very unequal ; in some localities many of them occur, in others they are only 

 very sparingly represented, or even entirely absent. This mode of occurrence seems now 

 to be in direct proportion to the number of specimens of each species which are found in 

 each locality. In places where very great numbers of individuals of two species are 

 heaped together, transitional forms are more numerous ; in other localities, on the con- 

 trary, where two species occur, but not in great numbers, transitional forms are rare and 

 even entirely absent. This seems to indicate a circumstance which has not up to the 

 present been sufficiently taken heed of. In localities where two species in very great 

 numbers were crowded, there seems to have been great possibility for the production of 

 hybrids, whilst in other localities where the two were not so numerous few or no 

 hybrids were produced, and thus in the cases of contemporaneous species the transitional 

 forms might for the greater part be due to the hybridity. But the production of hybrids 

 cannot establish identity of species. It can thus, I think, very well be sustained that, 

 even if transitional forms exist, yet two fossils of different shapes might very well form 

 different species, and thus we come back to the old axiom that every form that can be 

 fixed and distinguished from others by description and figures, has to be considered as 



1 Dr. Waagen says, "M'Coy in his 'British Palaeozoic Fossils' thought proper to apply the name 

 Seminula, which he had formerly given to other shells, to species which form now the genus Dieiasma ; 

 but, as in the original diagnosis of Seminula, only those shells were comprised which belong to the 

 families Rhynchonellidce and Spiriferidce, it does not seem advisable to transfer the name now to shells 

 belonging to the Terebratulidce ; thus, from this consideration also, the name Dieiasma will have to stand. 

 Bayle adopts the name Seminula for T. hastata, but from the above-given reasons the name will have to be 

 erased." 



