10 Herbert L. Hawlcins — Studies on the Echinoidea. 



two types of girdle characterize independent lineages. Hchinocyamus 

 (and the Scutellidse) may perhaps claim the small Biscoides as their 

 ancestors, while the genealogy of Clypeaster leads back through 

 Conodypus to such a type as Biscoides cylindricus. 



The essential difference between the Holectypoid and Clypeastroid 

 girdles lies in the apparent absence of any originally interradial 

 element in the latter. This element is, however, so persistently 

 minute in the Holectypoida that its suppression would be a slight 

 modification when compared with other morphological changes 

 introduced in the Tertiary Order. In the Clypeastridae, the virtual 

 exclusion of the interambulacral plates from the inner rim of the 

 peristome renders the change inevitable. In the Echinocyamus series 

 the fusion of the two transposed processes would tend to crush the 

 ridges out of existence, altliough it is at least conceivable that some 

 trace of them may remain in the middle of each compound " auricle ". 

 Except in the family of the Pibulariidae, the neighbourhood of the 

 peristome is practically free from structures due to secondary 

 thickening, so as to allow free play for tlie recumbent lantern. 

 There is thus no reason nor opportunity for the development of 

 a false ridge. The processes of Clypeaster are upheld by caringe 

 recalling those of Plesiechimis. 



If the false ridges of Biscoides or Conulus were removed (or laid 

 parallel with the floor of the test), and the already almost negligible 

 true ridges abandoned, the resulting girdle would be, for all intents 

 and purposes, that of the Clypeastridse. Further knowledge of the 

 ontogeny of the girdle of the Fibulariidse, Laganidse, and Scutellidse 

 is needed before a definite opinion can be formed as to the origin of 

 the interradially placed fused "auricle". As far as appears at 

 present, this could have been produced either by the convergence of 

 the isolated processes of a Clypeasttid, or independently by an inter- 

 radiad encroachment of the processes of an Holectypoid upon the 

 ridges, resulting in the destruction or incorporation of the latter. 

 In any case it seems clear that both types of Clypeastroid girdle can 

 be regarded as modifications of that of the Holectypoida. 



4. The Nucleolitoid Peristome, 



The following section is based upon an examination of twenty-seven 

 specimens of Nucleolites scutatus from the Corallian of Marcham, near 

 Abingdon, and of two examples of Trematopygus from the Faringdon 

 Greensand. These are the only forms of this Order in which I have 

 as yet succeeded in exposing the interior of the test. Serial sections 

 through the adoral regions of a specimen of Galeropygiis agariciformis 

 and two of Clypeus sinuattis seem to show no serious difierence in this 

 character from the smaller and more satisfactorily studied genera. 



The greater part of the test of the adoral surface of most 

 JSTucleolitidee is very thin, and the peristome is markedly invaginate. 

 (An exception to this rule occurs in those forms which have the 

 beginnings of a floscelle developed.) Seen from within the peristome 

 it resembles a truncated and obscurely pentagonal hollow cone. The 

 interambulacral parts of the margin are quite obviously thicker than 

 the ambulacral, and than the rest of the adoral part of the test. 



