536 DK. A. W. KOWE ON THE GENUS MICRASTER. [Aug. 1 899, 



Hanoverian form, reseniblmg E2naster gihhus in shape, but possessing 

 a well-marked fasciole. Having failed to obtain an example of this 

 variety, it is impossible for me to discuss it. I can only say that, while 

 the low-zonal gibbous forms reach their maximum elevation in the 

 broad and narrow examples from the Micraster cor-testudinarium- 

 zone, I have never seen any English specimen so tall and gibbous 

 as that figured by Wright, save in the high-zonal series. In this 

 sub-group, the dominant profile-shape is that with the flat-arched 

 upper surface (the Ooldfussian type), just as it is in the group of 

 M. prcecursor. 



Distribution. — This variety is not often found in the base of 

 the Bolaster 2:)lanus-zojie, but it is of moderately frequent occurrence 

 in the rest of that zone, including the Chalk-rock, when that bed 

 exists. It is found throughout the zone of M. cor-testudinarium^ 

 and in the base of the zone of M. cor-anguinum ; but after that 

 point it rapidly ceases to be the M. cor-testudinarium of Goldfuss, 

 and it becomes merely the var. latior of M. cor-anguinum auctorum 

 of the high zones. At Dover it is just as frequently found in the 

 //. plamis-zone as in its own zone, and in this section the largest 

 examples of all were collected immediately above the Chalk-rock. 

 At Beachy Head, Beer Head, and on the Dorset coast, the pro- 

 portion of broad forms in the H. planus-zone is not so large as 

 at Dover, The last-named locality, however, must stand as our 

 criterion, as no other section affords such opportunities for working 

 this zone, nor is any other so rich in Micraster. 



Allusion will be made in the next group to the limit-line of this 

 variety, which is fixed at the top of the lower third of the 

 M. cor-anguinum-zoiie, when it will be seen that no merely arbitrary 

 division-line has been made between M. cor-testudinarium and M. cor- 

 anrjuinum var. latior, but that the separation is warranted by fact. 



It is not a common form in the low-zonal series, and probably 

 20 per cent, would represent the frequency of its occurrence in the 

 H. planus- and M. cor-testudinarium-zones. In the base of the 

 M. cor-anguinum-zone the percentage is a little higher.^ 



The purist may object to this massing of the varieties of the 

 broad series, and say that as Goldfuss gives a figure of a moderately 

 elevated, flat-arched form, such as that shown in PI. XXXV, line v, 

 fiig. 2, the title of M. cor-testudinarium should be restricted to this. 

 The only answer to such an objection is that every low-zonal 

 species or variety has its definite scale of shape-variations, whether 

 it be broad or narrow, and that it is impossible to create a special 

 restriction in favour of one variant, as it at once breaks into the 

 scheme of zoological continuit}', which Micraster so well exemplifies. 

 The figure quoted is slightly flatter than the Goldfuss type, but it 

 would be easy to replace it by one of somewhat greater elevation. 



Affinities and differences. — Its complete accord with 



P Since writing the above I have visited the Dorset coast, and in one locality 

 there noted the unusual occiirrence of a M. cor-testudinarium-zcne where the 

 broad iornis predominated in numbers. This was only in one spot, for in other 

 exposures in the same zone the usual preponderance of narrow forms was the 

 rule.— «7«we 7th, 1899.] 



