520 DE. A. W. ROWE ON THE GENUS MICEASTEE. [Aug. 1 899, 



Shape. — Unlike that of any other Micraster. The large forms 

 are generally of an elongated ovoid shape ; the small are markedly 

 wedge-shaped, as a rule. Wright's figures are particularly good, 

 for they bring out the rows of prominences on the lateral aspect of 

 the test, which resemble the bossing on Hemiaster Morrisi. This is 

 seen only in large examples. 



Length and breadth. — It is a very long form, the length being, 

 as a rule, proi)ortionately greater than in any other Micraster. 

 Examples have been measured which are 7 mm. longer than broad, 

 and also broad examples, where the difference in favour of the 

 length is as little as 2 mm. ; but this small difference is unusual, 

 and a conspicuous length is the rule. As in all other species and 

 varieties, there is a broad form and a narrow form, but the writer 

 has never foand one with the length and breadth equal. Measure- 

 ments of three individuals and the average of small forms are here 

 given : — 



Length. Breadth. Height. 



1 79 mm. 73 mm. 49 mm. T. gracilis-zone, Dover. 



2 81mm. 76 mm. 44 mm. H. planus-zone, Du-ver. 



3 76 mm. 65 mm. 45 mm. Forbes's type. 



4 51 mm. 47 mm, 31 mm. Average ofsaiall forms, Dover. 



Height. — All the specimens of this species that I examined are 

 notably depressed. 



Position of apical disc. — This is remarkably anterior, 

 giving an average excentricity of no less than 9 mm., or 3 mm. 

 more than the average of all the other forms in the zone of 

 Holaster planus. No other Micraster shows any approach to this 

 anterior excentricity. 



Position of mouth. — Always distant from the anterior 

 margin, the average distance, for large and small forms combined, 

 being over 9 mm., and that for the small forms alone being just 

 under 9 mm. In large examples the month looks very small for 

 the size of the test. 



Peristome. — This is in the highest degree characteristic, con- 

 sisting of a smooth, flat ring, with no trace of the jointing of the 

 anterior peristomal plates. It has a striking resemblance to that 

 in Hemiaster. (PL XXXIX, fig. 1.) 



Lab rum. — Always present, but invariably rudimentary. It is 

 merely an angular downward-curving thickening of the peristomal 

 ring. The whole of the mouth-opening is exposed to view. This 

 is also similar to that in Hemiaster. (PI. XXXIX, fig. 1.) 



Paired ambulacra. — These are deep — notably deep when one 

 considers the thinness of the test, and very short for the size of 

 the test. The posterior pairs are flexuose, contracted at their 

 extremities, and much shorter than the anterior, the number of 

 pores being 23 to 28 in a medium- sized example. The paired 

 pores diff'er from those in other forms of Micraster, in that both 

 the inner and outer rows are elongated, and the two are 

 of almost equal length. In well-preserved examples, the outer 

 rows of pores are tear-shaped. So far as the writer knows, no 



