532 IE. A. W. EOWE ON THE GENUS MICRASTEE. [Aug. 1 899, 



of the last-named species, the slope of the posterior truncation is 

 of DO value as an index of species. Mic^mster Miclielini has not 

 been found in the English Chalk. 



Here, then, we have, from the base of the Holaster planus-zone to 

 the top of the lower third of the M. cor-angui num- zone (\h.e low-zonal 

 series of this paper), a collection of associated forms which vary 

 largely within certain definite limits, though in each zone we can 

 plainly trace the radiation from the several prominent variants just 

 mentioned. Whatever variation in shape M. prcecursor may assume 

 in the zones of H. 'planus, M. cor-testudinarium, and the base of 

 M. cor-anguinum, the general low-zonal features of the test are 

 shared by it in common with the other species and varieties in 

 these zones. The low-zonal features are enunciated on p. 516. It 

 is only the special features of the test that vary from zone to zone. 



So much, then, for the family resemblance, which is so marked 

 and continuous that the only way to handle the series is to form 

 them into a group, in which are contained the several persistent 

 and accentuated variations. But, in spite of this family resem- 

 blance, we can in the field allocate each one at a glance to its own 

 zone ; for so powerful have been the conditions of existence that every 

 variation, however marked, is impressed with characters special to 

 the zone in which it is found. A more striking tribute to the 

 value of zonal geology it would be impossible to conceive. 



The practical outcome of this postulate of fixed essential fea- 

 tures of the test for each zone is that we must either make 

 these prominent profile-variations of the M. prcecursor-gvovi]) do duty 

 in each of the zones in which they may occur, or we must face 

 the reductio ad absurdum of giving to each form a new specific name 

 for each zone in which it may be found. 



Take the forma Normannice by way of example (PI. XXXY, 

 Jines ii & iv, No. 1). As writers have described these variations on 

 the basis of shape alone, and ignore the essential zonal features, it 

 follows that we must retain this idea of well-known profile-forms, 

 and weave them into the scheme of zonal and zoological continuity. 

 This particular profile-form is found in the zones of H. planus^ 

 M. cor-testudinarium, and in the base of the zone of M.co7'-anguinurn, 

 and that in an abundance second only to the dominant flat-arched 

 form. To all outward appearance this type is the same in each zone, 

 and it is only when we come to examine the essential features of 

 the test that we learn that the form varies in its details 

 according to the zone in which it exists. 



When we realize that this same profile-form is found in M.cor-hovis 

 and M. Leskei, that it extends even to the high-zonal M. cor- 

 anguinum, and that it is not confined only to these narrow forms, 

 but embraces the broad forms in each zone — then we see of how 

 little value is mere shape in forming a species. Surely it is more 

 reasonable to recognize that there is a forma Normannice of this 

 group, occurring in the zones quoted, and varying zone by zone in 

 essential features, rather than to dub it by a different specific name 

 for each horizon at which it occurs. 



