'Vol. 55.] DK. A. W. HOWE OIST THE GENFS MICEASTEK. 509 



fig. 2) ; above MicrasUr LesJcei the granulation is present in an 

 ever-increasing degree. 



General shape of the test.' — The overwhelming importance 

 which most writers attach to this point is obvious from the nature of 

 the many ' species' figured, for in not a few instances a profile- view 

 only is given, and, where the base is shown, it is generally drawn 

 to demonstrate the contour of the ambitus and the position of the 

 peristome. In the majority of instances it would be impossible to 

 determine from the figure from which zone a specimen is derived, 

 and unless a figure tells us this fact it is practically valueless, for a 

 Mic7'aster which is not accurately zoned is not worth the trouble of 

 picking out of the Chalk. 



"We know that in living forms the mere profile varies greatly, 

 even within the limits of a definite species ; therefore it is difficult 

 to understand why, merely because the creature happens to be pre- 

 served in the fossil state, a simple depression or elevation of the test 

 should suffice for the erection of a specific type. 



If a notably depressed form such as M. Normannicp, Bucaille, of the 

 Holaster ^lanus-zojie, for instance, has other characters which mark 

 it out from the regular facies of the genus Micraster of that zone, 

 then, by all means, make it a species ; but if a flattened shape is its 

 only attribute, and if the same shape is found in forms from neigh- 

 bouring zones, then it is clear that a species has been coined on 

 insufficient grounds. To show the utter futility of manufacturing 

 species in this manner, PI. XXXV reproduces a photograph of a 

 graduated series of Micraster of diff'erent varieties and species, all 

 accurately zoned, arranged in rows on glass shelves. 



Allusion has been made before to the fact that there is a 

 broad and a narrow form of each species, though M. cor-hovis and 

 M. Leskei never yield examples in which the length and the breadth 

 are quite equal. Take the forms from the H. planus and M. cor- 

 testucUnarium-zones^ as shown in profile on PI. XXX Y, and it 

 will be seen that we can construct a complete scheme of varying 

 shape for each species or variety, be it of the long or broad type, 

 and that this scheme will apply to every species described in these 

 zones. 



It may be desirable at this point to give a full explanation 

 of PI. XXXV. In the first place, the fossils are photographed so 

 as to be as nearly as possible half the natural size ; for instance, 

 the large M. cor-hovis, at the right end of line ii, is exactly 80 mm. 

 long. This example is included, so that a comparison may be made 

 with the giant variety figured by Forbes and the much commoner 

 small type (line i, figs. 1-4). The lines must be inspected from 

 below upwards, in the order of the Chalk-zones. (It should be 

 noted that some of the examples of M. cor-hovis and M. Leskei were 

 found in the upper part of the Terebratulina gracilis-zone) : — 



