183 



read in the eloquent chapter xx., to which reference has abready been made in 

 this paper; and in a postscript suggested by the newly-pubUshed "Recherche. 

 Paleontologiques" of Barrande, he says that the large development of Cephalopods 

 in the SUurian period, and their rapid decrement in succeeding paleozoic forma- 

 tions, are facts "strikingly confirmatory of his estabUshed geological postulate, 

 that with the exception of its youngest member, the Silurian system was an 

 ' Invertebrate period' of immensely long duration" (page 506). The extreme 

 uncertainty of this argument has been shown elsewhere in the paper on " Upper 

 SUurian Fossils." It is not quite clear, however, what our author means by a 

 geological postulate ; if he means that it is axiomatic, that is, partaking of the 

 nature of a self-evident theorem, that fish did not exist in the earlier SQurian 

 seas, I cannot forbear expressing my grave doubts as to the accuracy of his 

 theory. The respect and gratitude we owe to the learned and painstaking 

 explorer of SUuria need not bind us to an unhesitating adherence to aU his 

 generalisations. 



Before we consider what is axiomatic in paleontology and what is not, two 

 preliminary observations are necessary. Since the last edition of "Siluria" 

 appeared, fish life has been brought down to the Lower Ludlow rocks ; now 

 does not Sir R. Murchison make too Ught a matter of the difference between 

 the Upper and Lower Ludlow ? Lower and Upper chalk may have been deposited 

 successively on the same deep sea-bed without osciUations of level or geogra- 

 phical changes, and it would be no great event to find in the lower beds a 

 species recognised already in the upper. But such is not the case with the 

 Ludlow rocks : the Upper Ludlow grey shales graduate imperceptibly into the 

 Old Red Sandstone, and it would have required a very acute geologist to point 

 out in the Ledbury tunnel the precise spot where the Silurian system ended 

 and the Devonian began, and the ichthyoUtes in these uppermost beds might 

 have been spoken of as the heralds of the numerous and varied Devonian shoals ; 

 but in passing down to the Lower Ludlow, we retrace, in the Ludlow promon- 

 tory at least, a history, which in its beds of various lithological characters, 

 alternations of limestone and shale, tells of many geographical and hydiogra- 

 phical changes. In spite of the nomenclature, is not the move from the Upper 

 to the Lower Ludlow as marked as that from the Lower Ludlow to the 

 "Wenlock Shale woxild be? Murchison himself says— "In a general sense, the 

 Ludlow rocks of the Silurian region of England and Wales must be simply 

 viewed as a continuation of the argillaceous masses which prevail in the under- 

 lying Wenlock formation. Such is more particvdarly the case in the lower 

 beds of this deposit," (p. 123), and in the next page, speaking of these very 

 Lower Ludlow rocks, he says that his " chief reason for grouping them with 

 the Ludlow rather with than with the Wenlock deposit was, that throughout 

 the typical districts of Shropshire and Herefordshire these shales occupy the 

 base of the ridges, the harder summits and outward slopes of which are com- 

 posed of Aymestry bmestone and Upper Ludlow rocks." I think then you wiU 

 agree with me that Sir Roderick's theory of the non-existence of fish in the 



