264 H. L. Hawlcins — Structure of Phyllodes in Fossil Echinoidea. 



second series is one which retained the more primitive features of 

 Galeropyyus, with but slight modifications, for a much longer period. 

 This was the faniiry of the JSTucleolitidaB (Echinobrissidse), in which 

 the ambulacra only attain a sub-petaloid condition adapically and 

 a hypophyllodul condition adorally. . Echinobrissus is a characteristic 

 genus of this group. But while, as far as the evidence seems to point, 

 the Gly peus-Pygurus group did not survive the Cretaceous period, the 

 Echinobrissus group underwent fresh differentiation in that period, 

 the petals becoming moi-e pronounced and the phyllodes highly 

 specialized. The resulting forms are typified by C 'atopy 'gun and 

 Echinolampas. 



The product of this Cretaceous development from the Nucleolitidse 

 is so like that of the Jurassic descendants of Galeropygus, on the 

 Clypeus line of descent, that both series have been grouped together 

 as the Cassidulidse. If the interpretation here indicated be correct, 

 this family will need subdivision into, approximately, the Jurassic 

 and Cretaceous Series respectively as distinct families, and would 

 serve as yet another illustration of the principle of ' heterogenetic 

 homceornorphy '. The heterogeneity is, however, more apparent than 

 real, as the Echinolampas series is the outcome of the slow develop- 

 ment of the Echinobrissus series, proceeding along the same lines as 

 those more rapidly traversed by the cognate Clypeus series. In 

 passing it is of interest to note that the group which quickly attained 

 a high specialization of petals and phyllodes was comparatively short- 

 lived, while the other group, which appeared at the same time, but 

 which did not reach the Clypeus stage of ambulacral complexity until 

 the Cretaceous period, is represented at the present day by members 

 of its simple (Nucleolites) and complex (Echinolampas) sections. 



There yet remains the problem of the origin of the Clypeastroids 

 and Spatangids. In both these large groups some ambulacral plate- 

 crushing occurs (e.g. in Clypeaster, Arachnoides, and Echinocardium), 

 but it never occurs on the adoral surface and is not always triserial. 

 I have reason to believe that this crushing in the petaloid parts of the 

 ambulacra is a secondary development, independent in origin from 

 that of the other Echinoids. The difficulty is encountered when one 

 considers the simplicity of the ambulacra on the adoral surface, where 

 surely the ancestor or ancestors of the groups must have possessed 

 more complex structures. At present, although from other considera- 

 tions the Clypeastroids seem to show affinity with such a form as 

 Disco'idea, and the earlier Spatangids with the ' Cassidulidte ', I have 

 found no evidence to indicate the way in which demiplates may be 

 restored to a primary condition, and caused to undergo such 

 remarkable changes in proportion as they show in these groups. 

 Bather (1909, p. 107) discusses the problem with regard to Orthopsis, 

 but comes to no definite conclusion. More evidence on the post-larval 

 development of recent forms is necessary before this question will be 

 ripe for solution. 



In conclusion I wish to express my indebtedness to Dr. E. A. Bather, 

 F.B.S., for his advice and assistance. 



Summary. — The physiological value of the phyllode is considered 

 to be both mechanical and sensory, and the structure of its plating 



