452 S. L. Hawkins — Tuberculation of Holed ypoida. 



are oblique, as well as the adradial ones. But while in Anorthopyyus 

 there are never more than the necessary two tubercles in the oblique 

 line, in Conulus three, or even four, may appear. The couple nearest 

 the median series is, however, usually only dual in its composition. 

 There is a greater tendency for this multiplication on the adradial 

 than on the interradial tracts, and the oblique series are more closely 

 packed in the former area. A few scattered supernumerary tubercles 

 always appear near the ambitus, but these are not so numerous, nor 

 so largely developed, as in Biscoidea. 



III. THE TREND OF EVOLUTION IN THE TUBEECULATION. 



In the descriptions of characteristic types just completed, there are 

 two features which stand out pre-eminently as fixed and uniform. 

 These are, the smooth, uncrenelated character of the summits of the 

 tubercles, and their occurrence in regular vertical series which appear 

 and disappear towards the two poles of the test in inverse order, 

 leaving one central and persistent series which alone occupies some of 

 the latest formed (adapical) and most reduced (peristomial) inter- 

 ambulacral plates. The universality of this unituberculate character 

 of some of the plates throughout the group is an indication of its 

 primitive position among the Irregular Echinoids, a belief which is 

 confirmed by the fact that the number of plates which retain this 

 character is less in the Cretaceous than in the Jurassic species. With 

 the possible exception of the Eehinonidoe and some of the genera allied 

 to Caratomus, it would seem that, in adult specimens, no plates with 

 a single tubercle remain. 



But against these two persistent features may be placed two of 

 equal importance, but exhibiting a progressive character when traced 

 through the group. These are the relative proportions of the tubercles 

 of the upper and lower surfaces of the test, and the distribution and 

 number of the tubercles on any individual ambulacral plate. In these 

 features the main lines of change seem to be maintained with 

 considerable regularity Avhen regarded stratigraphically. In the 

 Pyyastcrs, and especially in those from the Inferior Oolite, the 

 distinction in size between the tubercles of the two surfaces s hardly 

 appreciable. There is a greater density of packing adorally, but little 

 actual difference in the diameter of either scrobicule or boss. In the 

 later (Upper Jurassic) Pyyasters this similarity is less pronounced, 

 the adoral tubercles showing a marked increase in size, both actual 

 and relative. In Ifolectypus the change is carried further. The 

 number of adorally situated tubercles decreases, but their size increases 

 in an almost inverse proportion in If. depressus, while the adapical 

 tubercles are small and often imperfectly developed. In Biscoidea the 

 tendency reaches its maximum, a few series of large tubercles existing 

 adorally, while adapically the tubercles and granules can hardly be 

 distinguished from one another. Conulus, aud still more Anorthopyyus, 

 seem not to respond to the tendency in so marked a degree, but, as 

 these genera do not appear to have any direct genetic relationship 

 with Biscoidea, but rather to be an offshoot from Upper Jurassic 

 Holectypi or even Pyyasters, this fact does not interfere with the 

 general principle. 



