II. L. Hawkins — Tuberculation of Holeetypoida. 453 



In the peculiar Cenomanian form, Anorthopygus, the tuberculation 

 lias reverted (or at least shows a likeness) to the original Pygaster- 

 feature of equality all over the test. This archaic trait seems to 

 accord well with the retention of the periprOct on the ad apical surface. 

 Conulus follows with a fresh tendency to irregularity, more of relief 

 than of diameter, in the development of the tubercles. It may be 

 noted that this slight contrast in size is also met with in the very 

 closely allied genus Pyrina, the character of whose tuberculation 

 seems, in most respects, intermediate between that of Anorthopygus 

 and Conulus. 



In the details of tubercle-structure the change would appear to have 

 been from the concavo-convex outline of a boss standing freely in 

 a large scrobicule, to the purely convex outline of a boss rising almost 

 directly from the scrobicular circle. Coupled with this change is 

 a lessening of the relative size of the mamelon, and a shallowing 

 or even partial atrophy of the scrobicule. 



When the arrangement of the tubercles is considered, a far more 

 striking progression becomes apparent. In a young Pyg aster semi- 

 sxdcatus many of the interambulacral plates support but one tubercle 

 each, while there are usually three or tour, often more, plates at each 

 pole, which, even in adults, remain unituberculate. The arrangement 

 of the additional tubercles is different on the two sides of the median 

 row, being horizontal interradially and oblique adradially, the adradial 

 tract of the area thus becoming comparatively densely tuberculate. 



In Holectypus the number of additional tubercle series increases, 

 but always on the same plan of aiTangement as in Pyg aster, and 

 leaving the interradial tracts relatively bare of tubercles, a feature 

 betokening the persistence of a tendency most strongly marked in the 

 Regular Echinoids. Biscoidea carries on the sequence, but amplifies 

 and elaborates it by the addition of many extra tubercles at and near 

 the ambitus. 



This group of forms, which is typified by the three genera Pygaster, 

 Holectypus, and Biscoidea, may be taken as forming, broadly speaking, 

 a genetic series. 



In the case of Anorthopygus and Conulus a great difference is found. 

 Anorthopygus, although resembling a Pygaster in many of its features, 

 shows not merely an increase in the number of tubercles on each plate, 

 but a uniformity of distribution of those tubercles by the change from 

 the horizontal to the oblique order in the disposition of the interradial 

 series. In addition to this, an acceleration in the growth of the 

 tubercles appears, very few plates, whether adapical or adoral, 

 retaining the primitive unituberculate habit. In Conulus we seem to 

 have the same tendency to reduce the relative size of the adapical 

 tubercles, which appeared in the Pygaster-Biscoidea series, working 

 on an Anorthopygus-like arrangement of the tubercles. It seems 

 impossible to maintain a natural separation between the ' Echinonidse ' 

 and ' Echinoconidae ' in the feature of tuberculation, as in most other 

 details of structure. 



IV. SUMMAEY. 



It may be taken that the general tendency of the evolution of 

 surface ornament (and therefore of the radioles) in the Holeetypoida 



