260 H. L. Hawkins — Structure of Phyllodes 



The genus in this, as in many other features, shows a close affinity 

 with the Nucleolitidse, the only important difference being the 

 absence of any sub-petaloid development of the adapical regions of 

 the ambulacra. 



Tkematopygus faringdonensis, Wright. PI. XIII, Pig. 5. 

 Lower Greensand. 



Although this sub-genus is separated from its Jurassic relatives by 

 a very great interval of time, yet, in default of satisfactory specimens 

 of any of the Echinobrissi from the Oolites, I have chosen it as 

 a representative of the Nueleolitidas (Echinobrissidse). From the 

 scanty traces of sutures that I have been able to discern in 

 JSchinobrissus scutatus from the Corallian, there does not seem to be 

 any essential difference in the ambulacral structures. The peculiar 

 character of the incrustation upon the fossils from the dark-red beds 

 of the Faringdon Sponge-gravels renders the sutures, however fine 

 they may be, readily traceable. 



There is no appreciable expansion of the ambulacra towards -the 

 peristome, and only a trifling amount of depression. The adapical 

 portions are sub-petaloid, but from the apical system to some distance 

 beyond the ambitus the plates are primaries of varying vertical 

 diameter. The actual crushing-point is postponed until a slightly later 

 position than in Galeropyyus, but anticipatory demiplates (usually of 

 series c) sometimes appear spasmodically, though they never occur on 

 the adapical surface. Where the pore-pairs become definitely triserial, 

 the plates are crushed in a manner exactly comparable with that 

 described in Galeropygus, but plate a is sometimes separated from the 

 interradial margin by a considerable space. A corresponding simpli- 

 fication of the structure is seen in the immediate vicinity of the 

 peristome. 



Galeropygus and Trematopyyus may, judging by the somewhat 

 scanty evidence I have been able to find in other genera, be taken as 

 representing the typical condition of phyllode development in the 

 Nucleolitidae. On the analogy of the term sub-petaloid as applied to 

 the adapical regions of ambulacra, I suggest the term hypophyllodal to 

 express the condition of the adoral parts of ambulacra which have the 

 true phyllode structure as regards their platiug, but which are not 

 expanded or depressed. 



Clypetjs ploti, auct. PI. XIII, Fig. 7. Inferior Oolite. 

 The most striking distinctions between Clypeus and Galeropygus are- 

 found in the extreme petaloid character of the adapical parts of the 

 ambulacra in the former, coupled with the development of a marked 

 floscelle with prominent bourrelets and broad phyllodes near the 

 peristome. In C. ploti the ambulacrals retain their primary character 

 until the expansion for the phyllodes begins. Then plate-crushing 

 rapidly supervenes ; but, although plate c is the first to become 

 compressed, it rarely altogether becomes reduced to a demiplate, while 

 plate a becomes almost, if not quite, separated from the interradial 

 margin from the first. Apart from the persistence of the primary 

 character in plate c, the plating is quite similar to that in the hypo- 

 phyllodal forms already described. There is, however, an advance in 





