142 THE FOSSIL ECHINOIDEA 



of the anterior and posterior slope of the longitudinal profile, arising from the com- 

 paratively less tumid character of the posterior portion of the test — this variation 

 being accompanied, in specimens from localities ii. and iii. (Survey-numbers ^|p and 

 ^^), by a slightly greater prominence along the median line of the posterior odd 

 interradium, amounting almost to a faint rostration at the posterior margin. 



The greatest amount of modification from the normal type occurs in specimens frona 

 locality ix. (Survey-number ^^). This form appears so well marked that we pro- 

 pose to distinguish it nominally as var. conicus. It differs from the typical A. subro- 

 tundus in the following points : — The proportion of the length to the breadth is greater 

 than in the type, the apex is proportionately higher, and the anterior portion of the 

 marginal contour is more contracted and less bombous in the area circumscribed by the 

 anterior interradia. Seen in longitudinal profile, both the posterior and the anterior 

 slope form much steeper declivities, the curvature of the posterior being only very ' 

 slightly more bombous than the anterior one. From this it will be seen that the test 

 has a subconoidal appearance. The margins are less thick, the distance between the 

 periproct and the peristome is perhaps greater than usual, and the position of the peri- 

 stome more excentric, whilst that of the apical disk is subcentral. Unfortunately the 

 specimens are much weathered on the abactinal surface, and not well preserved. 



Along with the above notices of variations, mention should also be made of two 

 examples which we find amongst a series of small (young) specimens. The tests are 

 unquestionably immature, but differ from the others in the higher and conoidal cha- 

 racter of the dorsal area, in the margin being less thick and tumid, and in the peculiar 

 subpentagonal marginal contour, expanded opposite the anterior petals, and contracted 

 posteriorly, where the odd interradium is subrostrate. Possibly this may prove to be 

 the young of a different species ; but for the present we do not feel justified in drawing 

 any definite conclusions, in consideration of the immature state of the specimens. 



There are also portions of three or four tests from locality viii. (Eri Hill, Survey- 

 number -^f^) which are only referred to this species with considerable hesitation ; they 



may possibly belong to a distinct form. The fragments are well preserved, but are 

 insufiicient for positive determination. 



Premature Form. In young specimens of 40 millim. length the test is suboval in 

 outline, with very thick margins and an almost regular convex dorsal area, which is 

 generally higher over the whole surface than in the adult — the height at the apex being 

 proportional to the length as 0-50: 1. The ambulacral petals are well developed and 

 present the characters of the mature form. With increasing age and size, the height 

 does not increase proportionately with the other dimensions of the test, and the margins 

 become also relatively less thick. In a specimen 58 millim. long the regular convex 

 curve of the dorsal surface, the strikingly pulvinate character of the actinal area, and 

 the thick tumid margins are very characteristic. In this specimen the proportion of 

 length to height is as 1 : 0-43. It is not until the test is fully grown that the almost 

 circular form of the marginal outline is attained, and the posterior portion of the test 

 becomes more tumid and generally higher than the anterior part. 



