C. J. A. Meijer — Micrasters in the English Challi. 115 



— speaking roughly, some thousands of feet of elevation above — 

 2000 ft. depression below — 600 ft. again elevation above the present 

 level, in the north ; and that the south should have shared in the 

 first and the last of these movements, as it must have done, for 

 England to be united with the Continent, but that it should have 

 had no corresponding share in the intermediate one ? Surely this is 

 highly improbable a 'priori. There cannot be any magic in the 

 line between the mouth of the Thames and the Bristol Channel 

 that it should present an effectual obstacle to the continuation of a 

 movement which must have been due to deep-seated and wide- 

 spread causes operating within the crust of the earth. 



IV. MiCRASTEES IN THE ENGLISH ChALK TwO OK MORE SpECIES ? 



By C. J. A. Meyer, JF.G.S. 



TO the student of Cretaceous palceontology there could be suggested, 

 probably, no more puzzling question than that of the determina- 

 tion of the species of the genus Micraster. It seems not unlikely 

 that Professor Forbes was of this opinion when, in 1850,' he reduced 

 to varieties of a single species — that of Micraster cor-anguinum — no 

 fewer than seventeen previously supposed species, or subspecies, of 

 the genus. It is true that, out of compunction, perhaps, for such 

 wholesale slaughter, he forthwith established one new species, the 

 Micraster cor-bovis, and pointed out the probable existence of another 

 — " a small Micraster with a very elevated extremity from the Chalk 

 of Lyme." This, however, seems scarcely to make up for the 

 absorption or obliteration of all but one of the previously described 

 species. And, seeing that several of these so-called varieties of 

 Micraster cor-anguinum are recognized as species by many conti- 

 nental palaeontologists, it becomes an interesting subject for inquiry 

 whether our English Chalk does not contain more than one species 

 in addition to the recognized Micraster cor-anguinum and cor-bovis ? 



The genus Micraster, as defined by Agassiz, appears to have met 

 with very general acceptance. Its stratigraphical range is brief. 

 Its extreme of variation, although considerable, is not great as com- 

 pared with that of many other genera. Whence then has arisen so 

 wide a difference of opinion in respect to the number and value of 

 its species ? 



This question has not unfrequently occurred to me when seeking 

 some palpable distinction between specimens of Micraster seemingly 

 different. The answer appears to lie in the smallness of the differ- 

 ence between one species or specimen and another. 



It is embarrassing, certainly, to find that in a given series of 

 specimens one should feel compelled to recognize either several 

 species or variations only of a single species according to the mode 

 of arrangement of the specimens, and yet such appears to be practi- 

 cally the case. Arrange the specimens irrespectively of strati- 

 graphical considerations, as Forbes appears to have done, and it is 

 possible to obtain such a graduated series as shall apparently 

 obliterate all marked distinctions of length, breadth, and height. 

 1 Mem. Geol. Surv. decade iii. 



