430 Correspondence — R. M. Brydone. 



COI?.I?,E;SI=03SriDE3SrOE!. 



MICBASTER PRJECURSOR, EOWE. 



Sir, — Your reviewer of The Stratigraphy of the Chalk of Hants 

 in the March Geol. Mag., 1913, pp. 122-3, seems to have misunder- 

 stood my remarks on this species; at any rate, the views from which 

 he appears to be dissenting are not views advocated or held by me. 

 I should therefore like to restate the points I intended to raise. 



In the first place it appears to me that Dr. Howe's monograph in 

 effect established that every individual of the species M. prcecursor 

 possesses two sets of characters, which liave practically nothing in 

 common. The first set embraces a large number of characters which 

 the individual shares with all specimens, not only of M. prcecursor, 

 but also of M. cor-testudinarium (and sometimes other species also) 

 from the same horizon ; these characters are therefore of great zonal 

 value, but no specific value. The second set embraces a small number 

 of characters which enable M. precursor to be distinguished from its 

 nearest ally, 31. cor-testudinarium. As neither of these species is 

 confined to a single zone, this second set of characters are not strictly 

 of any zo7ial value ; i.e., a record of M. prcecursor does not identify 

 the zone, but it only tells you that you are in one of two or (under 

 Dr. Rowe's zonal classification) three zones. To put it briefly, the 

 assemblage of individuals defined as the species M. prcecursor has no 

 zonal value ; the individuals have, through characters which are not 

 specific, great zonal value, (I did not expressly affirm the latter 

 proposition because I regarded it as established beyond dispute ; 

 but your reviewer seems to have failed to perceive that the two 

 propositions are quite independent, and to have assumed that 

 a challenge of the general view on one must necessarily be a challenge 

 of the general view on the other.) 



Secondly, on examining the characters by which M. prcBcursor is 

 to be distinguished from M. cor-testudinarium they appear to consist 

 substantially of shape variations in a particular direction, that of the 

 proportion between length and breadth, with concurrent variations in 

 minor features necessarily affected by variation in general shape. 

 That, is to say, the species M. prcecursor is based almost entirely upon 

 shape variations, while the prime object of Dr. Rowe's monograph 

 was to prove that shape variations were not a valid basis for species 

 of Micraster. This seems to indicate that M. prcecursor is not 

 specifically separable from M. cor-testudinarium as defined by 

 Dr. Rowe, but that they are two sections of a single species which 

 obviously must be known as M. cor-testudinarium, and that prcecursor 

 should be suppressed as a specific name. This does not in the least 

 prejudice the zonal value of the individuals hitherto comprised in the 

 species M. prcecursor. 



Thirdly, it is legitimate to inquire whether the prcecursor section 

 of the species M. cor-testudinarium is a natural one, for which 

 'prcecursor^ can usefully be retained as a varietal or other subsidiary 

 name. It seems to me that if it is a natural section, the dividing-line 

 employed by Dr. Rowe ought to occupy the lowest, or nearly the 

 lowest, point in a curve of frequency plotted for the various proportions 



