198 Herbert L. ffaivkins — Studies on the Echinoidea, etc. 



axis of the tubercle (which, is at right angles to the surface of the 

 test in normal cases) slopes away from the apex downwards towards 

 the ambitus (see PL XIII, Fig. 3). There seems to be almost perfect 

 homogeneity among the sunken tubercles, apart from their variation 

 in size. 



On each of the first few plates from the apical system there is 

 only one "sunken tubercle" (Fig. 1), constantly situated near the 

 adoral transverse suture of the plate, and nearer to the interradial 

 suture than the tubercle of the central (Cidaroid) series. This 

 particular row of "sunken tubercles" can be distinguished, with 

 but little variation in relative position, on all the interambulacral 

 plates of the adapical surface except those immediately above the 

 ambitus. Its members are commonly larger than the other less 

 regularly disposed tubercles of the same type. The number of 

 "sunken tubercles" on each plate shows a steady increase until just 

 above the ambitus, but the depth of their areolae shows a corresponding 

 decrease. They never become very numerous. Although none of 

 the additional "sunken tubercles" show any constancy of position 

 comparable with that of the first-mentioned series, it is a general 

 rule tli at the largest and most deeply excavate ones are situated 

 near to, and sometimes in contact with, the adoral transverse margins 

 of the plates. 



A comparison of Figs. 1, 2, and 4 (PI. XIII) discloses an interesting 

 evolutional feature in connexion with the "sunken tubercles". In 

 Holectypus depressus (the Cornbrash form, not that from the Inferior 

 Oolite) there is, one such tubercle on each of the interambulacral 

 plates just above the ambitus, but never more than one. The plates 

 near the apex are without any. This solitary "sunken tubercle" 

 is in precisely the same relation to the central primary as it is in the 

 adapical plates of D. cylindricus. Thus the "young" plates of the 

 Cretaceous Discoides show the same development, in this respect, as 

 the " adult " plates of the Jurassic Holectypus. The evidence for the 

 phyletic continuity between the Holectypinse and Discoidiinee thus 

 receives a convincing addition. 



(b) The Ambulacra of the Adapical Surface. 



The ornament of the ambulacral plates is on a smaller scale, and 

 less thickly spread, than that of the interambulacrals, but it consists, 

 none the less, of a corresponding series of five different structures. 

 The primary tubercles are, on the whole, very regularly arranged in 

 vertical series on every third plate. The secondary tubercles are 

 few, and are usually associated with either a primary areola or 

 a peripodium. The miliaries are very small, but are relatively 

 numerous, and for the most part form transverse lines across the 

 middle of each plate. 



The two peculiar types of ornament are both present, the " raised 

 secondaries" in fair numbers, and the "sunken tubercles" with 

 much the same frequency for a given area as those of the inter- 

 ambulacra. I have never seen more than one " sunken tubercle " 

 on an ambulacral plate. As in the interambulacra, there is one 

 vertical series of these tubercles (situated near the peripodia) which 



