SEA-UKCUINS. 475 



among the otlier members of the order. Although suggesting a 

 comparison with the large interpetalous interambulacral tubercles 

 of such a genus as Eupatagus, it seems to be a specific character 

 of no genetic value. 



3. 27ie Miliary Granules. 



Tu r)jgaster, and to a less degree in II olectyj) us, the granulation 

 shows a " Regular " nffinity in being grouped around the primary- 

 tubercles to form scrobicular rings. This circul.-ir arrangement is 

 not retained after the Jui'assic period. In Ilolectypvs, especially 

 in //. dfpressKS, the scrobicular miliaries on the adradial tracts of 

 the plates near the peiistome are often guttate in form. In 

 Macropygns and Avorthopygus the large size of the scrobicules of 

 the primary tubercles leaves little room for miliaries, but those 

 which exist do not appreciably group themselves around the 

 primaries. In Discoidea and Cmnholectypits, and to a slight 

 degree in Ilolectypus sens, str., rows of granules radiate from the 

 central primary tubercle, maintaining a more or less transverse 

 direction, to reach the adradial and interradial margins of the 

 plate. In the genus first named these linear rows of granules 

 become interspersed, nenr the ambitus, with additional tubei'cles. 



In Comd'us the granulation is apparently without a definite 

 .system of arrangement. The granules are sunk slightly below the 

 level of the test on the adapical surface, being enclosed in minute 

 pits. On the adoral surface they regain their projecting chai-acter, 

 and often occupy broad bands, slightly elevated, which correspond 

 in position with the transverse sutures of the plates. 



F. The Radioles. 



Our knowledge of the acanthology of the Holectypoids is 

 fnigmentarv and inadequate. Enough is known, however, to 

 show that there exists a considerable uniformity in the character 

 of the radioles throughout the group. Wright (20) has described 

 the primary radioles of Pygaster, in the species semisidcatus and 

 (Macropygns) morrisii. For both he uses almost the .same words — 

 short, needle-shaped bodies with fine longitudinal lines on the .stem. 

 I have not seen any specimens in which they are preserved. In 

 the case of Ilolectypus there is a specimen of H. depressus (from 

 the Inferior Oolite of Cheltenham) in my collection which I'etains 

 a considerable number of radioles on both the adapical and adoral 

 surfaces. Wright {t. c.) describes them in the same species. The 

 primaries of the upper surface ai'e very shoit and slender, with 

 l>lunt tips. Those of the adoral surface were apparently quite 

 long, and but slightly tapering. The collars of the adoral radioles 

 are prominent, and often very oblique. The shafts of both sets 

 of radioles are longitudinally fluted with closely-.set ribs. The 

 miliary granules support spines of a similar character to the 

 adoral primaries, but far more minute, so that Wright's descrij)- 

 tion of them as " hair-like " is accurate. 



