Professor Bonney — Pebbles in the Trias. 75 



Note. — It is time some examination was made of the well-worn 

 argument with regard to the " unchangeable persistence of the 

 Bracbiopoda " once more repeated by that brilliant and popular 

 winter, Prof. Heni-y Drummond, who is supposed to reduce the hard 

 facts of science to the mental pabulum adapted to modern social 

 innocents (Ascent of Man, 1894). The facts respecting this 

 " unchanged persistence " and the percentage of survivals will be 

 duly presented in a textual and tabulated form. Hence, it will 

 now suffice to state that out of 47 families only seven survive. 

 One genus out of the 24 included in the ancestral order Atremata. 

 persists, from the Lower Silurian upwards, and that is Lingiila. But 

 this "persistent" Lingtda was preceded by a number of antecedent 

 forms many of which died out before its generic characters were 

 established. Three genera out of 31 members of the divei'gent 

 order Neotbemata still survive; only one, Crania, "persists" from 

 the Lower Silurian upwards. Both these are relatively shallow 

 water forms, mostly rock-fixed and shore haunters. The sole survivor 

 out of 82 genera of the order Protremata belongs to a secondary 

 (Thecidoid) type. The remaining known genera, about 140 in 

 number, of which somewhat less than thirty survive, all belong to 

 two out of the three main subdivisions of the fourth order of 

 Telotremata, which was scarcely differentiated in the Lower 

 Silurian age. Taking Mr. Schuchert's careful estimate of genera 

 (tabulated March, 1893), we find that, at a rough estimate, two 

 Palaeozoic genera out of 175 all told "persist." Thus the rate of 

 " unchanged persistence " among the Brachiopoda is less than one 

 per cent, of the generic mutations evolved — or, say one per cent., 

 making full allowance for generic differences determined by Hall, 

 Clarke, Beecher, and others since that article was issued. Neither 

 of the so-called " persistent genera " appears in the primordial fauna, 

 and the recent species of both these composite genera bear unto 

 this day visible traces of their obolleloid and paterine Cambrian 

 ancestors. So much for the unchanged "persistence" of the Brachio- 

 poda. Eadical stocks, as Pi-ofessor Hyatt has recently shown {Joe. 

 cit. 44*), may continue to be represented, but not by "persistent" 

 identical generic types. Among the Tetrabranchiate Cephalopoda 

 the result is even more marked than among the Brachiopoda, as 

 the generic term Nautilus is strictly applicable only to recent 

 species, and possibly to some Tertiary offshoots of the main Nautiloid 

 persistent stem and primordial radical. — A. C. 



(To be concluded in our next Number.) 



IV. — A Comparison of the Pebbles in the Trias of Budleigh 



Salterton and of Cannock Chase.^ 



By Professor T. G. Bonney, D.Sc, LL.D., F.R.S., etc. 



THE pebble-bed in the Trias of Budleigh Salterton has attracted 

 much notice, but I am not aware that its materials have been 

 compared with those of the corresponding bed in the Bunter group 



^ A paper, read in substance before Section C, at the Meeting of the British 

 Association at Oxford. 



