506 DR. A. W. EOWE ON THE GENUS MICRASTEE. [Allg. 1 899, 



on the anterior end of tlie labral plate. In all high-zonal forms, 

 on the contrary, the tubercles are continued almost to the end ol 

 the labrum, and secondary tubercles and granules at the extreme 

 end of the labrum are the rule. 



The shape of the labral plate in low-zonal forms is broadest where 

 it joins the plastron, and it tapers to a point towards the labrum, being- 

 roughly triangular (PI. XXXVII, fig. 8). In the high-zonal forms, 

 on the other hand, the labral plate is, as a rule, narrowest at the 

 plastron al end, keeping a nearly equal width until it approaches 

 the labrum, where it notably expands (PI. XXXVII, fig. 9). 



The arrangement of the tubercles on the labral plate is generally 

 irregular in the low-zonal forms, and more symmetrical in high- 

 zonal forms, tending in the latter to constitute a regular multi- 

 columnar series ; moreover, in Micraster LesJcei there is a hiatus 

 between the plastronal and the labral tubercles, and the same 

 statement applies to the narrow ' sutured ' forms, however large 

 they may be, which form the transition-series between M. LesJcei 

 and the Holaster planus-ty^e of M. 2:>rcecursor. In the II. planus- 

 types of M, prcecvrsor and M. cor-testudinarium^ on the other hand, 

 we find in a quarter of the number a merging of the plastronal and 

 labral tubercles into a continuous series ; and in the same forms, in 

 the M. co7'-testudinarium-zone, the proportion in which the sets of 

 tubercles merge becomes one third ; while in the high-zonal forms, 

 in the top of the zone of M. cor-anguinum, the proportion reaches 

 nearly a half. 



In M. cor-hovis the tubercles are grouped towards the labral tip, 

 and are very few (PI. XXXIX, figs. 2-5) ; in M. Leskei they are 

 also few and irregularly placed along the length of the labral plate 

 (PI. XXXYIII, fig. 1); in the narrow 'sutured' transition -forms 

 between 31. LesJcei and 31. prcecursor, of the R. planus-zone, the 

 tubercles are more abundant and more symmetrical ; and the 

 symmetry and abundance increase in the M. cor-iestudinariuyn-zone. 

 The paucity and asymmetry of the series of tubercles in M. cor- 

 hovis and M. LesJcei are not evidences of immaturity, as they are 

 found in the largest specimens of the two species ; while, on the 

 contrary, in the smallest examples of other forms in the H. planus- 

 zone both the shape of the labral plate and the arrangement of 

 the tubercles are quite diflperent. The importance of this will be 

 seen when we come to deal with M. LesJcei as a separate species. 



Tip of labrum. — This is a very characteristic feature. In all 

 low-zonal forms there is a tendency for the tip to be smooth, and 

 invariably it is strongly granular in all high-zonal forms. 



In M. cor-hovis and M. LesJcei the labrum is rudimentary, and 

 merely a projection from the smooth-ringed peristome. In the 

 narrow ' sutured ' transition-form between M. LesJcei and the 

 II. planus- tyipe oi M. prceciu'sor it is invariably smooth; whereas, 

 in the M. cor-testudinarium-zona it is slightly granular in a third of 

 the forms ; and in the upper part of the zone of M. cor-anjuinum it 

 is strongly granular in every instance, the granulation extending 

 even to the upper surface of the labrum, inside the test. 



