Vol. 55.] DR. A. W. ROWE ON THE GENUS MICRASTER. 501 



The subdivided and strongly inflated types are characteristic 

 •of the zone of Micraster cor-testudinarium, but they are also seen in 

 the lower part of the i¥. cor-anguinum-2one. The subdivided form 

 is more strongly represented in the M. cor-testudinarium-zone at 

 Beachy Head than it is in the same horizon at Beer Head and Dover, 

 but the difference is not sufficient to alter the conclusions herein 

 .stated (PI. XXXVI, fig. 6). 



The divided area is never found in the iToZas^^r ^knws-zone, very 

 •occasionally (barely 3 per cent.) in the M. cor-testudinarium-zoTie, 

 in about 20 per cent, of the forms in the lower third of the M. cor- 

 .anguinum-zone, while above the latter level it becomes the common 

 form. This type differs from the subdivided variety by reason of 

 the completeness of the furrow between the central tumid portions 

 •of the ambulacral plates, which separates the base of the inflations as 

 well as the top, giving a vertical central trench, just as if a cut had 

 been made with a saw. 



At the first glance, the divided area in the zones of M. cor- 

 iestudinarium and M. cor-anguinum differs considerably, but in 

 reality the method of division is the same in both, and the apparent 

 lack of similarity is due to the fact that the ambulacrum in the 

 former is deeply excavated, and that in the latter it is shallow. 



At this point it may be useful to note that the general shape 

 ■of the ambulacra varies in the different beds, for in the zones 

 •of T. gracilis and H. planus, where the smooth, sutured, and 

 gently inflated areas predominate, the general form of the 

 ambulacrum is that of a flat trough ; while in the M. cor-testudi- 

 .narium-zone, and in that of M. cor-anguinum (lower third of zone), 

 the excavation is deeper, more strongly sloped, and much more 

 angular. As soon as we pass above the low-zonal limit, we find 

 that the ambulacra become much shallower, so that in many instances 

 they are almost flush with the surface of the test. 



It is worthy of mention also that there is a progressive scale of 

 increase in the size of the interporous ridge and of its granules, 

 till, when we reach the high-zonal forms, we find the linear series 

 of granules very prominent. In connexion with this feature it 

 should be remarked that in the low-zoual forms the linear granules 

 •cease abruptly at the inner row of pores, whereas, in the high-zonal 

 forms they are continued on to the interporiferous plate, curving 

 round the whole margin of it in the form of a hook (PI. XXXYI, 

 ;fig. 7). A foreshadowing of this last-mentioned condition may be 

 found in one or two granules which curve round below the inner 

 row of pores. This feature is seen in Pi. XXXVI, fig. 5, which 

 represents a specimen derived from the M. cor-testudinarium-zonQ. 



While dealing with the paired ambulacra of the superior surface, 

 it may be of interest to refer to the ambulacra of the inferior 

 ;surface. Five avenues (double rows of paired pores) radiate from 

 the mouth — two posterior, two lateral, and one anterior ; and two 

 jgingle rows are found just inside the short vertical arms of the 



