240 Prof. P. M. Duncan and Mr. W. P. Sladen on the 



of Palceostoma are not present in the form we named Hemi- 

 aster elongatus, nor are they in the closely allied species H. 

 digonu3. 



VI. 



Prof. Lov^n remarks in his work on Pourtalesia^ p. 75 : — 

 " Tn the adult of Hemiaster and of all the other Spatangoids 

 of early Mesozoic origin the calyx in the adult presents a 

 structure essentially different from that of the calyx in the 

 adult of a Spatangus or of any other form of later or recent 

 appearance, and very rarely a link is found between the two." 

 On page 73 of the same admirable work there is the following 

 statement in reference to Hemiaster : — " Now these three 

 species of the existing seas are true HemiasterSj fully sharing 

 the well-known characteristics of that highly natural genus, 

 the subglobose form, elevated in its posterior region, a calyx 

 of four costals [generative plates], the madreporite confined 

 to costal 2, the 4 and 1 and the radials I. and V. closing 

 from either side, in strict accordance with the mode of con- 

 formation universally prevalent within the calycinal system 

 during the older Mesozoic period." 



Loven then compares this calicular or apical arrangement 

 of the Hemiasters with that seen in the genus Ahatus. This 

 he shows has five generative plates, " the 5 being repro- 

 duced between the radials I. and V., and bearing the madre- 

 poric filter, thus set free, on its unimpeded retrograde move- 

 ment." He asserts that if a form like this is allowed to 

 remain in the genus Hemiaster its integrity is vitiated. 



Under any circumstances the two forms we have described 

 as Hemiasters are closely allied to Ahatus^ but they have no 

 basal (" costal ") plate 5, and the madreporite is clearly on the 

 plate 2 ; moreover, the Sindian forms never have three gene- 

 rative pores. Ahatus has the same arrangement of the inter- 

 radial plates on the actinal surface in areas 1 and 4 as the 

 two Hemiasters from Sind ; the plates of the ambulacrum 

 No. I. are more numerous in Ahatus than in our Hemiasters'^ 

 but the shape of plate 1 of interradium 5 of both forms is the 

 same. Comparing the Sindian Hemiasters with the recent 

 H. expergitus, it will be noticed that the details of the actinal 

 surface correspond ; but on the apical disk the madreporite of 

 the recent form does not pass between the plates 1 and 4. 

 The correspondence is not very close, however, in dimensions, 

 for the ambulacra on either side of interradium 5 are broader 

 in the recent form, and the front part of the peristome is not 

 •with a prominent rim. Considering the two species to belong 



