258 H. L. Haivkim — Structure of Phyllodes 



collection is given in PL XIII, Pig. 1. The pores are seen to be 

 roughly triserial in arrangement, but the innermost member of eacli 

 triad is considerably separated from the other two. In Echinolampa.s 

 the pores are single. In some genera (e.g. Catopygus) only the inner 

 series contains single pores, the outer two series being composed of 

 pore-pairs. Although I have been unable to find any evidence on the 

 subject, I consider it at least possible that the inner series of single 

 pores may represent sensory tentacles, while the closely crowded pore- 

 pairs of the outer series give passage to ordinary adhesive podia. 



Although the arrangement of the pores is usually easily traceable, 

 it is commonly very difficult to distinguish the plate-sutures near the 

 peristome. By a process seemingly analogous to the obliteration 

 of sutures in the skulls of adult vertebrates, these earlier-formed 

 ambulacral plates tend to become very closely united. Perhaps the 

 mechanical crushing which takes place in the adoral parts of the 

 ambulacra may contribute to this effect. In fossil forms the difficulty 

 is often enhanced by the development of crystalline cleavage-cracks, 

 and accidental fractures. A certain type of weathering, by which 

 the surface of the test is destroyed and all the sutures widened, is 

 serviceable in some specimens, and in default of this a microscopic 

 section will, when viewed under crossed nicols, indicate the position 

 of the separate plates \>y the varying orientation of the calcite of 

 which they are composed. In every case it is essential that the 

 suturing be studied near the outer surface of the test, as there is often 

 a considerable difference in the degree of plate-crushing developed 

 on the inner and outer regions, that of the inner being usually less 

 pronounced. 



The arrangement of the pore-pairs and the tendencies of the plate- 

 crushing are definitely triserial in the peristomial regions of all the 

 forms I have examined, whether recent or fossil. This arrangement 

 is particularly interesting in view of the fact that the plate-crushing 

 in the Holectypoida, the most primitive group of Irregular Echinoids, 

 is always triserial, in whatever part of the ambulacra it may occur. 

 I have recently described this feature in outline (Hawkins, 1910, 

 p. 349), and this paper may be regarded as a sequel to that research. 

 The notation of the plates used here in the descriptions and diagrams 

 is the same as that employed in the paper referred to, and the diagram 

 there given is copied, with additions, on PL XIII, Pig. 2. 



The phyllode is usually regarded as a structure restricted to the 

 CassidulidaB (including the Xucleolitidse), but this is only true of its 

 most complete development. Wherever, in Irregular Echinoids, the 

 pore-pairs show a tendency to become triserial towards the peristome, 

 the elements of phyllode structure seem to be present (e.g. Conulus). 

 Prom the simple and incipient compression of the ambulacral plates in 

 Pggaster, every degree of complexity of structure is traceable up to 

 such an elaborate phyllode as that of Catopygus. Only in the 

 Clypeastroids and Spatangids (among Irregular Echinoids) is no 

 tendency to phyllode development to be found. 



Certain charactezastic stages in the formation of phyllodes will now 

 be described, and then the phylogenetic significance of its development 

 will be discussed. 



