508 DR. A. W. HOWE ON THE GENUS MICRASTER. [A.Ug. 1 899, 



Length of ambulacra. — There is a progressive increase in the 

 length of the paired ambulacra as one ascends in the Chalk-zones. 

 Micraster cor-bovis has them remarkably short, and they are short 

 also in M. LesJcei ; after that the increase is gradual. M. hrevis, 

 Desor, would be an exception to this rule ; but the writer has 

 never found it in England, nor has he met with any record of its 

 ■occurrence. 



Curve of posterior ambulacra. — The curve is notable in 

 M. cor-bovis, less in M. Leskei, and becomes gradually less until one 

 reaches the top of the M. co7'-anguinum-zoiie, where in a definite 

 percentage of narrow forms the posterior ambulacra are quite straight. 

 Exceptions will be found in the broad high-zonal forms, such as 

 M. cor-anguinum var. latior and M. glyphus. 



Interporous ridge. — This may be absent in M. cor-bovis, and 

 is always feeble in M. Leskei ; after these two marked forms die 

 out there is always a progressive increase of the granulation of the 

 ridge in other forms as one ascends the zones. The extension of 

 the granulations on to the interporiferous area of the forms at the 

 top of the M. cor-testudina7num-zoTiQ and in the M. cor-anguinum^ 

 zone has already been mentioned (PI. XXXYI, figs. 5 & 7). 



Sub-anal fasciole. — Feeble in M, cor-bovis, and only mode- 

 rately developed in all low-zonal forms ; strong, on the contrary, in 

 high-zonal forms. In the low-zonal forms there is always a 

 tendency to a contraction in the short diameter, and in the high- 

 zonal series it is almost always broad and straight. The transition 

 from zone to zone is exemplified in PL XXXIX, figs. 5-7, and 

 the contrast clearly brought out. 



Intra-fasciolar bosses. — In high-zonal forms the narrow 

 posterior end of the base generally ends in two marked prominences, 

 which are situated within the fasciolar area. These are absent in 

 all low-zonal forms. 



Latero-oral ambulacral grooves. — Spreading out on either 

 «ide of the peristome are two grooves, each containing an avenue of 

 arched pores. These grooves are a marked feature of all low-zonal 

 forms, and are but slightly indicated in the high-zonal series. 



Inflation of plastron. — This is iU-marked in low-zonal 

 forms, and the same applies to the labral plate ; but in high-zonal 

 forms the inflation, in both cases, becomes more marked. 



Primary tubercles of base. — In M. cor-bovis the areolar 

 circle is made up of a scanty and scattered ring of large granules 

 (PI. XXXYIII, fig. 6) : in M. LesJcei it is much stronger, and offers 

 a certain guidance in determining the two forms (PI. XXXYIII, 



