II. L. Hawkins — Tuberculation of Holed ypoida. 449 



granules helps to give this part of the test an almost uniformly 

 granular appearance. In' the smaller species of the genus, sucli as 

 D. subucula, this feature is carried so far that, except in very well- 

 preserved specimens, it is practically impossible to distinguish between 

 the tubercles and the granules. In D. cylindrica the distinction is 

 rather more apparent. The main row of first-formed tubercles is 

 considerably more clearly marked than the lateral series on the upper 

 surface, and is situated on a prominence of the test which often 

 becomes a carina passing vertically clown the median line of each 

 interambulacral plate. On the adoral surface the tubercles are much 

 larger, and are all of approximately equal size. As will be seen by 

 reference to the figure, the structure of an individual tubercle is very 

 like that in the previously described genus, save that the basal terrace 

 is peculiarly prominent and the platform very broad. It should be 

 noted, however, that the adapically situated tubercles have practically 

 no scrobicule, a fact which renders their separation from the granules 

 additionally difficult. 



(b) The Arrangement of the Tubercles. (Fig. 2, F.) 

 The diagram for this species is an actual copy of the tuberculation 

 of one specimen — a beautifully preserved example of the forma 

 depresm — in the collection of Mr. G. E. Dibley, F.G.S. It was 

 found in the zone of Holaster subglvbosus at Burham, Kent. It will be 

 seen that the usual order of horizontal rows on the interradial, and of 

 oblique rows on the adradial, tracts obtains in this species. There is 

 a marked increase in number (as well as in size) of the tubercles at 

 and below the ambitus, but these very rapidly disappear when traced 

 towards the mouth, so that the adoral surface is but sparsely 

 tuberculate over most of its area. In addition to the increase in 

 size and number of the normal tubercles at the ambitus, many of the 

 granules (which are usually disposed in more or less transverse lines 

 in this genus) become exalted to the rank of tubercles, giving an 

 appearance of double horizontal rows of tubercles on each plate in 

 this region. Often these supernumeraries are quite as large as the 

 original tubercles, although, for the sake of clearness, they are 

 represented as dots in the diagram. 



7. Anorthopygus orbicularis (Grateloup). 

 (a) The Structure of the Tubercles. (Fig. 1, E.) 

 The most striking contract between the tuberculation in this genus 

 and that in those already described lies in the fact that the tubercles 

 of the adapical surface are hardly distinguishable from those of the 

 adoral surface in point of size. In this the genus recalls the sub-genus 

 Jfacropygus, but the equality in size is yet further noticeable than in 

 that Upper Jurassic series. The scrobicules are rather more deeply 

 excavate than in IZolecfi/pus, and the mamelons considerably larger 

 in proportion, but otherwise there are no important differences in 

 structure. 



(b) The Arrangement of the Tubercles. (Fig. 2, E.) 

 For a long time, even after I had become familiar with the order of 

 the tuberculation in Comilus, I was unable to satisfy myself as to the 



DECADE V. — VOL. VILT. — NO. X. 29 



