Yol. 55.] DK. A. W. ROWE 01^ THE GENTJS MICRASTER. 521 



Other Micraster has the outer pores of this shape. On looking at 

 PI. XXXVI, fig. 1, the left-hand rows of pores seem much rounder 

 and more crowded. This is not merely an effect of lighting and 

 foreshortening, for an examination of all the specimens shows that 

 the anterior wall of the ambulacrum in this species is much steeper 

 than the posterior, and the pores have thus much less space for 

 spreading out. The interporous ridge is either absent or feebly 

 developed. 



Single ambulacrum. — This is comparable with the paired 

 ambulacra, though, instead of being absolutely smooth, it is orna- 

 mented with fine and discrete granules. The only single ambula- 

 crum which in any way resembles it is that of M. Leslcei ; but in 

 the latter the granulation is more abundant and close-set. (PI. 

 XXXVII, fig. 1.) 



Interpo rife reus area. — This forms the type of the 'smooth ' 

 area. It is devoid of granules, and shows no trace of the suturing 

 of the interzonal plates. Owing to its smoothness and lack of cha- 

 racter, it is very difficult to photograph. It will be noticed that the 

 posterior ambulacra have a great resemblance to those of Hemiaster 

 Morrisi, for both are short, curved, contracted at the end, with 

 feeble or absent interporous ridge, elongated internal pores, and 

 smooth interporiferous area (PI. XXXVI, fig. 1). 



Anteal sulcus and notch. — Both are very shallow — so 

 shallow, indeed, as to be practically absent in some examples. In 

 this again the form now described resembles Hemiaster. 



Curve of superior surface. — This falls equally under two 

 groups — the flat-arched and the gibbous. On PI. XXXV, line i, 

 Nos. 1, 2, 3, & 4, are seen, in their order, the forma Normannice, 

 the fl.at-arched form, the form with the slight carinal rise, and the 

 gibbous variety. The writer has been unable to find the round- 

 arched or beonensis-iorm, though there is little doubt that it exists. 

 Eostrum and carina. — Eostrum invariably absent, and carina 

 never more than rudimentary. 



Sub-anal fasciole. — Oval and very weak — weaker than in any 

 other Micjrister in the low-zonal series. 



Plastron. — This is in the last degree characteristic. Tlie rows 

 of tubercles are very regular, and owing to the thinness of the test 

 they are very sharply cut, and stand out in a prominent manner. 

 This prominence is intensified by the fact that the tubercles rise 

 fiheer out of a broad, smooth, and flat areola, and that the areolar 

 circle is thin, flat, and scattered, made up of a few prominent 

 and very discrete granules. This is well seen in PI. XXXVIII, 

 fig. 6, and the contrast between it and that of 31. Leskei is sharply 

 brought out on fig, 2 of the same plate. 



Periplastroual area. — This is a broad, smooth, or very 

 faintly granular band, which surrounds the plastron and labral 

 plate, and by its smoothness throws up the plastron into still 

 greater prominence. This smooth area is constant, and is in 

 striking contrast to the strongly granular band which surrounds 

 the plastron in all other forms of Micraster. 



