504 DE. A. W. EOWE 0]Sr THE GENUS MICEASTEE. [Aug. 1899, 



Superior surface. — To facilitate reference, the forms which 

 slope evenly and gently, back and front (as in PI. XXXV, line iv, 

 fig. 2), are called * flat-arched ' ; and to those which have a sharp 

 and straight fall from apex to anus (as in PI. XXXV, line iii, fig. 6) 

 the term ' gibbous ' is applied, in contradistinction to those which 

 have a curved posterior fall, like that in Micraster Brow/niarti, 

 Hebert, in M. beonensis, Gauthier, and in M. intermedium, Bucaille. 

 The last-named shape is seen in PL XXXV, line i, fig. 3, and in 

 fact in fig. 3 of all the lines on that plate. It will be seen, on 

 reference to PI. XXXV, that these outlines, together with other 

 definite profiles, run right up through the zones, and a full survey of 

 them will be given when the plate is discussed in detail (p. 509). 



It is of interest to note that these types of superior surface are 

 restricted to no one species or variety, and that we see them 

 all exemplified in undoubted examples of M. cor-hovis, M. Leskei, 

 M. jjrcecursor, M. cor-testudinarium, M. cor-anguinum, and M. cor- 

 anguinum var. latior ; further, that we may find each or any of 

 these species with a slight posterior rise, thus foreshadowing the 

 strong carination, which is one of the features of the high-zonal 

 Micraster. The high-zonal forms are merely a repetition of the 

 low-zonal series, with greater development of special features. 



Another characteristic is that in the low-zonal forms the anterior 

 slope is gentle and rounded, and that in the high-zonal forms it is 

 very sharp. This sharp slope is well seen in the Gravesend 

 examples on lines vi and vii in PL XXXV : here, again, the same 

 progressive development is noted. 



Rostrum and carina. — In the Terehratulina gracilis-zone, in 

 the case of M. cor-hovis, both are absent ; the rostrum in the zone of 

 Holaster planus is absent in about 90 per cent. ; in the M. cor- 

 testudinarium-zone in about 80 per cent. ; in the base of the M. cor- 

 anguinum-zone it becomes more developed ; and in the high-zonal 

 forms it is a common feature. 



The carina in the H. planus-zowe is absent in 72 per cent., slight 

 in 20 per cent., moderate in 7 per cent., and strong only in 1 per 

 cent. ; in the M. cor-testudiiiarium-zone it is absent in 60 per cent., 

 slight in 32 per cent., and moderate in 6 per cent. ; in the base 

 of the M. cor-anguiymm-zone it notably increases, and in the high- 

 zonal forms it is one of the conspicuous features of the test. It is 

 worthy of note that the development of the carina is always in 

 advance of that of the rostrum ; the latter is, in fact, quite a late 

 development. 



Again we are compelled to notice a progressive evolution of a 

 feature from the low to the high zones. 



Apical disc. — The Tabular Summary (pp. 512-513) gives all 

 the necessary percentages, and shows a marked progressive dimi- 

 nution of the anterior excentricity. 



No attempt has been made to figure or describe the progressive 

 granulation of the disc, as this feature of the test is commonly 



