H. L. Haivkins — Echinoidea Holectypoida. 397 



petaloid, narrow, widest at the ambitus. Posterior petals longer 

 than anterior ; that of area III the shortest. Petaloid pores dis- 

 similar, outer pores elliptical and larger than the inner, round, 

 pores. Extra-petaloid pores single and small, three or four in the 

 peristomial invagination being slightly larger than the rest, but not 

 attaining the size of the outer petaloid pores. Pore-series irregularly 

 uniserial (with tendenciestotriserial arrangement belowthe ambitus), 

 convergent adorally. Proximal orad plates primaries (biporous 

 ambulacrals present) within invaginated j)arts. Thence to ambitus 

 typical " Pyrinid " plating, most intense (in part Discoidiid) in 

 areas I and V. Similar plating continued above the ambitus into 

 distal part of petal in area III, \ almost to jDetals in II and IV, 

 but not reaching above ambitus in I and V. In areas last-named 

 supra-ambital, non-petaloid plates are relatively high. Petals 

 (except in III) uniformly Cidaroid in plating. 

 Recent : New Zealand and 1 Madagascar. 



3. The Affinities of Apatoptgus. 



The entire facies of the test of Ajjatopygus agrees very closely with 

 that of Nucleolites, and is superficially almost identical with that 

 of Treynatopygus. The large size of the peristome, and the marked 

 disparity in length of the petals are obvious features that distinguish 

 it from the former, while the directly transverse elongation of the 

 peristome runs counter to the diagnostic character of the latter. 

 The ambulacral structure, with which this paper is mainly concerned, 

 clearly separates Apatopygus from Nucleolites, and approximates it 

 to Trematojyygiis. It is needless to argue further the close relationship 

 connecting all three genera. To my mind the original contention 

 that " Nucleolites " recens is a latter-day survivor of the essentially 

 Mesozoic Nucleolitoida is perfectly justified. 



In respect of ambulacral structure, it is possible to recognize 

 four types in the Nucleolitidae {sens, str.), each of which can be 

 ascribed to a more or less defined place in stratigraphical history. 

 The Lower Oolitic type (e.g. " Nucleolites " quadratus, see Phil. 

 Trans., vol. 209, pi. Ixviii, fig. 2) had well-marked, congested petals, 

 and strictly limited phyllodes containing many occluded plates. 

 The Upper Oolitic type (e.g. Nucleolites scutatus, loc. cit., pi. Ixviii, 

 fig. 3) had less-restricted, little-congested petals, and ill-defined 

 hypophyllodes in which occluded plates are rare, but demi-plates 

 may occur. The Cretaceous type {Trematopygus, loc. cit., pi. Ixviii, 

 fig. 4) had long, many-plated, but feebly expanded petals, and a 

 strangely diffuse, vestigial kind of hypophyllode, in which Pyrinid 

 plating covers much of the adoral extent of the ambulacra, being 

 least extensive in areas I and V. The fourth type is Apatop)ygus, 

 with long but uncongested petals, and an exaggerated state of 

 diffusion in the " hypophyllode ", which shows features normal 

 to Trematopygus developed to a far greater extent. The four types 

 surely illustrate a morphogenetic sequence. While petal-characters 



