500 DE. A. W. EOWE ON THE GENUS MICRASTEE. [Aug. 1 899,. 



nature, and tlie smooth area is never found above the upper limit 

 of the zone of Holaster planus — a point which is most useful when 

 working in the field, in the absence, or rarity, of examples oiH. planus. 

 All the areas in this class have the sutures so faint that they are 

 unrecognizable with a lens, acd even in the case of Micrader LesJcei 

 there is no perceptible granulation of the smooth central portion 

 (PI. XXXVT, figs. 1 & 2). 



The sutured area is confined to the zone of H. planus^ as- 

 44 per cent, are classified under this heading in that zone. It is very 

 occasionally found in the Terehratulina gracilis-zone, but never in 

 that of Micraster cor-testudinarium. The interporiferous area in this 

 class has its sutures more or less plainly visible with a lens, though 

 to the naked eye they generally look smooth. There is no ornamen- 

 tation of the plates, and at most they are only faintly granular, but 

 they are always perfectly flat. 



This type of area is chiefly observed in the passage-form from 

 M. LesTcei to M. pr(ECursor, and it is also found in many examples 

 of the M. cor-testudinarium that occur in this zone. The smooth 

 and sutured areas are, then, quite characteristic of the zone of 

 H. planus^ and are never found above the upper limit of that zone 

 (PI. XXXYI, fig. 3). 



The inflated area is associated with the subdivided form in' 

 the zone of M. cor-testudinarium, and these two forms are charac- 

 teristic of this zone. In this bed the inflation of the area is always 

 strong, and is in the form of coarsely granular pads, which meet and 

 dovetail in the middle line. About 50 per cent, are of this type. 



In the H. planus-zone 30 per cent, are of this nature, but the 

 inflation is much weaker and fades away into the sutured form, 

 giving a perfectly easy transition from the flat to the raised area in- 

 this single zone (PI. XXXYI, figs. 4 & 5). 



The subdivided interporiferous area is characterized by very 

 tumid and strongly granular plates, which slope sharply to the 

 middle line, thus giving a deep and angular ambulacrum. The 

 slope is interrupted at the inflation of the interporiferous plates, but 

 the fall is then continued again as sharply as before, giving the 

 appearance of a trench within a trench. To the naked eye it 

 looks as if the interporiferous plates were entirely separated in the 

 middle line, but examination with a lens shows that it is only the 

 tumid upper portions of the plates which are separated, and that 

 the bases are in actual contact. This description will sufiice for 

 all examples in the low-zonal series. In the high-zonal series, 

 however, the central furrow is not nearly so marked, owing to the 

 fact that the ambulacral walls are much less steep. In well- 

 preserved specimens it will be noted that the granules of the 

 interporous ridge pass on to the interporiferous area, as will be 

 demonstrated in the description of the interporous ridge. This is, 

 of course, an entirely high-zonal feature (PI. XXXVI, fig. 7). 



