Yol. 55.] DK. A. W. EOWE ON THE GENUS MICRASTEK. 533 



The special features of the test in this group may be enumerated 

 as follows : — 



Labral plate. — In the Ilolaster planus-zone it is triangular, 

 with the broad base at the plastron, and tapering to a point at the 

 labrum (PI. XXXVII, fig. 8); in the Micraster cor-testudinarium-zone 

 the base of the triangles becomes notably narrower, and the point 

 less tapering^ (fig. 7); in the base of the M. cor-angumum-zoTVQ 

 it becomes narrower still, and tends to expand at the labrum 

 (fig. 6) ; in the upper part of the M. cor-anguinum-zone the plate 

 is invariably compressed within two close parallel lines, which 

 notably expand at the labrum, and the rows of tubercles, instead 

 of forming a triangular series, assume a linear arrangement 

 (fig. 9). 



Labral tip and peristome. — In the H. planus-zone the 

 passage-form from M. LesJcei to M. prcecursor has the whole posterior 

 segment of the peristome smooth, and the other prcecursor-iorms 

 have only the labral tip smooth ; in the M. cor-tesiudinarium-zone 

 the labral tip is smooth in 80 per cent. ; in the base ol the M. cor- 

 anguinum-zoJie it is frequently granular ; while in the upper part 

 of the last zone (in the case of M. cor-anguinum auctorum) it is 

 invariably granular, even on the under surface of the labrum. 



Interporiferous area. — As a rapid aid to determination in 

 the field this is useful in the last degree, and even when the base 

 of the specimen is broken, one can tell the zone from which it is 

 derived at a glance. It is ' sutured ' (never smooth) and feebly 

 ' infiated ' in the R. planus-zouQ ; strongly ' inflated ' or ' sub- 

 divided ' in the M. cor-testudinariam-zone ; and ' subdivided,' or 

 more generally ' divided,' in the top of the M. cor-anguinum-zone. 



Shape of ambulacral groove. — The transverse section is 

 that of a rounded trough in the H. planus-zone ; in the M. cor- 

 testudinarium-zone, that of a deep triangular excavation ; in the 

 base of the M. cor-anguinum-zone the triangular excavation is 

 becoming shallower ; and in the upper part of the last zone (high- 

 zonal) it shallows to the extent of being almost flush with the test 

 in many examples. See the transverse sections in PI. XXXVI. 



Periplastronal area. — A progressive degree of granulation is 

 observed throughout the low zones, culminating in the mammillated 

 prominences in the high -zonal forms, at the top of the M. cor- 

 anguinum-zone. (PI. XXXVII, figs. 6-9.) 



Young state. — Somewhat tumid, but not always. Ambulacra 

 and notch shallower. 



Affinities and differences. — The associations with il/. X^sAra 

 have been set forth in the description of the passage-form from that 

 species into the M. prwcursor of the H. planus-zone. With 

 M. cor-testudinarium, as regards its special features of the test, 



1 Many Micrastersinthe M. cor-testudinartum-zoue have a triangular labral 

 plate. While the two forms of plate are found in this zone, the narrow form ia 

 never found in the H. planus-zone, and therefore, where it is doubtful in 

 which of the two zones a specimen occurred, the narrow plate would definitely 

 exclude the latter zone. 



