446 H. L. Hawkins — Tuberculation of Holed ypoida. 



Spatangids. The platform arid mamelon are like those of P. semi- 

 sulcatus, save for their smaller proportions to the rest of the tubercle. 



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

 Though fewer in number and less regular in size, the tubercles in 

 P. umbrella follow the same plan of arrangement as do those of 

 P. semisulcatus. On the zigzag series of the adradial tracts, a curious, 

 but apparently constant, irregularity in the order of the appearance of 

 the vertical rows occurs. After the proximal series of adapically 

 situated tubercles has appeared, the next to develop is series three, 

 also adapical. Not for a plate or two do these two series become 

 separated by the delayed appearance of the adorally placed series two. 

 The same anomaly occurs in the growth of the next pair, and the 

 retention of the principle of inverse order for their disappearance on 

 the adoral surface results in a similar irregularity of the series in 

 that region. 



The incoming of irregularity in the tuberculation seems somewhat 

 significant in view of the fact that the crowding of the tubercles 

 is less marked in this species than in any others of the genus. It is 

 not surprising to find trifling disorder appearing when the tubercles 

 are minute and numerous, but in this case there seems no other reason 

 for its presence but some fundamental tendency in evolution. Pileus 

 pileus, which is also a Corallian form, and closely allied to Pygaster, 

 seems to show a similar tendency, although I have not been able 

 to examine any specimens which show the tuberculation clearly. 

 Pygaster macrocyphus, from the Kimmeridge Clay, undoubtedly has an 

 exactly similar irregularity, and it may be that these late Pygasters, 

 with the periproct passing farther and farther from the apex, are 

 showing in the tuberculation that plasticity of development which 

 often accompanies or precedes the differentiation of new types. 

 It would, however, be premature to discuss here the possible 

 outcome of this reawakening of evolution in the genus. 



3. Pygaster 'morrisi:', 1 Wright. (Fig. 1, C.) 

 The group to which P. laganoides, P. gresslyi, and P. truncatus belong 

 is one which was separated from the true Pygasters by Pomel {Classi- 

 fication methodique, 1883) under the generic or sub-generic name of 

 Macropygus. Previously both Desor (Synopsis) and Cotteau (Echinides 

 de la Sarthe) had expressed the opinion that the group, while admittedly 

 possessing peculiar features, did not merit generic distinction. Little 

 support or comment has been given to Pomel's resuscitation of Desor's 

 disowned name Macropygus, but the considerable differences from the 

 typical forms that the three species referred to above exhibit might, 

 perhaps, warrant its retention in a sub-generic sense. In no particular 

 is this difference more clearly shown than in the character of the 

 tuberculation. Instead of the shallow, broad scrobicules of P. semi- 

 sulcatus, these parts in P. (Macropygus) ' morrisi' are narrow and deep, 

 so deep that the top of the mamelon is often but little above the 



1 I do not wish here to express an opinion as to the distinction or identity 

 between this species and P. laganoides, Agass., but use Wrigbt's name to 

 indicate tbat the specimens I have examined are all from British localities. 



