Correspondence — Prof. T. G. Bonney. 475 



before the end of the Devonian. The forms of this species are 

 extremely variable, and Professor Gosselet gives in this Memoir the 

 results of the study of numerous specimens, more particularly of 

 those occurring in the Schistes de Barvaux, where it is especially 

 numerous and well preserved. Detailed measurements of a large 

 series of forms show important variations in nearly every feature of 

 the shell, and the only constant character is the simple form of the 

 ribs on the lateral or winged portions. The extreme variations are 

 connected by a crowd of intermediate forms, and in the course of 

 development the same individual may pass from one variation to 

 another. There can, therefore, be no question of varieties in a 

 zoological sense, and the author arranges the various modifications 

 into groups of forms, six in number, taking as a basis the proportion 

 of the width of the shell to the length of the smaller valve. 



The author considers that S. Verneuili finally dies out without 

 giving rise to another species ; for though there are no extreme 

 differences between it and such forms as S. attenuatvs and S. 

 Mosquensis, there are yet no intermediate forms known to indicate 

 that it may be the ancestor of these latter. Discussing the question 

 of the name properly belonging to the species, that of Murchison is 

 adopted as, on the whole, having the best claim to priority. 



To all interested in the question of Brachiopodal development 

 the carefully worked out details in this Memoir, and its abundant 

 illustrations, should prove highly valuable. G. J. H. 



coiaiaiBSiF'oisriDiBi^rciE]- 



OEIGIN OF THE YOUNGER RED ROCKS. 



SiK, — In his very interesting paper "On the Younger Eed Rocks," 

 my friend Dr. Irving professes himself to be more than ever con- 

 vinced that the pebbly sandstones of the Bun ter— such as those in 

 Notts — consist of Triassic sandbanks deposited in narrow tidal seas, 

 and "the great pebble beds of the Warwickshire and Budleigh 

 Salterton type" are "the ' Chesil banks' which formed the shore 

 equivalents of the sandbanks." 



This is a matter on which I thirst for knowledge. Will he then 

 kindly inform me : (1) What instances of marine conglomerates are 

 known in which length, breadth, and thickness are simultaneously 

 great ? As a rule, the zone on which ordinary shingle is deposited 

 extends but a few feet vertically above high-water mark, and not 

 many below it. These conditions must somew^hat restrict its breadth, 

 and still more its thickness. Banks of the " Chesil " type can attain 

 a greater thickness (though I should be glad to hear of one which 

 came near or exceeded 100 feet), but they are comparatively narrow. 

 (Mr. H. B. Woodward states that the Chesil Bank at Portland is 

 200 yards wide and 42 feet high.) But, according to Mr. Ussher 

 and other authorities, the length and breadth of the area occupied 

 by the Budleigh Salterton pebble bed is measured by miles, and its 

 thickness varies from 50 to nearly 100 feet. The pebble beds in 



