Vol. 55.] DE. A. W. ROWE ON THE GENUS MICEASTEK. 529 



the flat-trough shape, and never triangular in section. The inter- 

 poriferous area is always strongly 'sutured,' as shown in PI. XXX.VI, 

 fig. 3. The transition between the perfectly flat ' sutured ' area 

 and the faintly ' inflated ' area begins in this form. 



Peristome. — The raised ring disappears, and becomes quite flat 

 and jointed, in the anterior segment, like all other low-zonal forms, 

 only the jointiug is not so marked (PI. XXXYIII, fig. 4). 



Lab rum. — It has always a smooth labral tip, however large the 

 example may be, whereas, in the other low-zonal forms, the tip 

 becomes invaded by granules, in a certain percentage of cases. 

 Further, the posterior segment of the peristome is always smooth, 

 while it is frequently granular in the other low-zonal forms, 



Labral plate. — This is of the same shape as in Micraster LesJcei 

 and the other Micrasters of the Holaster lolcmus-zone, but the tubercles 

 are much more numerous than in the former species, and often form 

 a triangular series of marked symmetry, and of considerable extent. 

 Further, in M. LesJcei, there is always a hiatus between the 

 plastronal and labral series of tubercles, whereas, in the passage- 

 forms the two sets are not infrequently merged into a continuous 

 whole. 



Shape. — The passage-form may assume any shape, in the same 

 way that M. prcecursor of the H. jplanus-zone does, and those 

 figured in PL XXXV, line ii, give an exact idea of what one may 

 expect. Another point of extreme interest is that the broad forms in 

 the H. planus-zone, which one may call M. cor-testudinarium — the 

 M, deciji)iens, Bayle, of Continental writers — frequently have ambu- 

 lacra so smooth that they fall within the ' sutured ' class, showing 

 clearly that this type of ambulacrum is an horizonal and not a 

 specific feature. 



Distribution of M. LesJcei and its passage-form. — At 

 Dover, Beachy Head, on the Dorset coast, and at Beer Head, 

 M. Leskei is found in abundance throughout the whole thickness of 

 the zone of H. planus^ but no example has been obtained from the 

 M. cor-testudinarium-zone. In the Terehratulina f/racilis-zone at 

 Dover it occurs at the top of that zone, but is very rare, and no 

 trace of it has been discovered at the same horizon at Beachy Head, 

 on the Dorset coast, or at Beer Head. At Beer Head alone, in the zone 

 oi Rhynchonella Cuvieri, it is found, but is quite rare. The passage- 

 form is found in abundance throughout the H. plnnus-zone, in all 

 three sections, but it has never been obtained below or above that 

 particular horizon. It will thus be seen that in M. Leskei and its 

 passage-form we have an unfailing guide for the top of the II. planus- 

 zone, and that, by its aid alone, we can always determine in the field 

 the zoological junction -line between this zone and that of M. cor- 

 testudinarium. Therefore, quite apart from its zoological interest, 

 it is of such value to the field-worker that it is worth placing on 

 record. 



It may be argued that this separation of allied forms is artificial, 

 and that the true M. LesJcei-forms may be looked upon as immature 

 examples of the larger transition-forms. Indeed, the writer at first 



Q.J.G. S. No. 219. 2 m 



