Ixii PROCEEDINGS OF THE (JEOLOGICAL SOCIETY. 



present stage in the world's history, — whether we consider the physical 

 conditions of the area from whence our data are derived, either under 

 a cUmatal or an orographical aspect, or tlie natural-history features 

 of the population of that area, — as any "ctage" of the upper or lower 

 Mesozoic period was from any proximate " ctage." Nay, the differ- 

 ence was even greater ; for the physical and natural-history characters 

 of the Pleistocene epoch that intervened were quite of as much differ- 

 ential importance as those of either "etage." Yet now that we know 

 the Crag MoUuscan fauna, we might almost say perfectly, no sane 

 naturalist can for a moment deny that a large proportion of the 

 species are positively identical with living types. Let those who 

 would hastily draw a hue of death between the faunas of proximate 

 " etages," and regulate their geological conclusions accordingly, 

 ponder well over this significant fact. 



The essay on the classification of the Brachiopoda, by Mr. David- 

 son, contains the conclusions arrived at after many years of conscien- 

 tious labour, mainly devoted to this interesting order of Mollusks, 

 for whose illustration we owe so much to his pen and pencil. No 

 other palseontologist has ever had so great an amount of perfect mate- 

 rials for his particular task at his command, and neither expense nor 

 labour has been spared by our indefatigable associate to render his 

 monograph as perfect as possible. If any of our brethren dissent 

 from some of his specific decisions, they must all admit that they 

 have been arrived at on no superficial grounds. The portion of !Mr. 

 Davidson's work now sent forth is entirely systematic, and is de- 

 voted chiefly to an exposition of the characters and definition of 

 the families and genera of Brachiopoda. He admits 33 genera 

 assembled under 10 principal families, with some inteiinediate and 

 doubtful or provisional groups. As he has endeavoured to define his 

 genera on the strictest natural characters, and appears to have suc- 

 ceeded in arriving at an arrangement, in the main sound and near 

 to the truth, it becomes an inquiry of considerable interest to ascer- 

 tain how far the ranges of these genera are continuous in time ; in 

 other words, whether the theory of unique generic time-areas be 

 borne out among the Brachiopoda, now that we may be said to have 

 attained so extensive a knowledge of their generic and specific types. 

 This was doubtless the idea working in the mind of Von Buch, when 

 with indifferent materials, he attempted to fix the characters ol' the 

 fossil Brachiopoda, and plainly has often influenced the numerous 

 attempts at their classification made by subsequent paleontologists. I 

 have no reason to suppose that an a priori hypothesis, connected with 

 either time- or space-distribution, influenced Mr. Davidson in coming 

 to his final arrangement, and therefore I have been the more curious 

 to see how far that arrangement accorded with geological considera- 

 tions. 



In the first family, TerebratuJicIce, the typical genus Terehratula 

 (of which Terehratulina and TT'ahUieimia are regarded as subgenera), 

 the succession of types is continuous from the middle palaeozoic or 

 Devonian epoch to the present time ; whilst the other genera are 

 either Upper Mesozoic, Tertiary, and recent (as Terebratella and 



