Ii. L. Hawkins — Structure of Phyllodes in Fossil Echinoidea. 257 



happily stated, from my point of view, the idea itself, that we must 

 admit the existence of two independent magmas from which all 

 igneous rocks originated by differentiation and assimilation, seems 

 to he a true one. 



(To be concluded in our next Number.) 



III. — On the Structure and Evolution of the Phyllodes in some 

 Fossil Echinoidea. 



By Herbert L. Hawkins, B.Sc, F.G.S., of University College, Beading. 

 (PLATE XIII.) 



IN all the Regular Echinoids, with the exception of the Cidarids, 

 and in the Irregular Echinoids other than the Clypeastroids and 

 Spatangids, there is a tendency for the ambulacral plates to become 

 crowded together as they approach the margin of the peristome. 

 This is the inevitable result of the continuous formation of fresh 

 plates at the edges of the oculars in the apical system, and their 

 less rapid resorption at the opposite extremity of the ambulacrum. 

 The compression of the ambulacral plates results in a crowding. of 

 the pore-pairs in the peristomial region, and this character seems to 

 serve a sensory as well as a motor purpose. 



In the Regular Echinoids the muscular effort of mastication is 

 liable to raise the whole body, supported on the teeth, away from the 

 surface on which the food is lying, and so the presence of an increased 

 number of adhesive podia around the mouth is a feature of consider- 

 able advantage. Among those forms of the Edentate Irregular 

 Echinoids which are not provided with a labrum, the importance of 

 keeping the mouth firmly applied to the food-bearing surface is yet 

 more urgent, and the increase in the number of the peristomial pores 

 becomes proportionately greater. At the same time, it is necessary 

 that some sensory power should be developed in this region, so that 

 the animal may get an estimate of the quality and character of the 

 food supply. Eor this function some of the podia would appear to be 

 specialized. In the Spatangids, for instance, the specially large pores 

 round the peristome give exit, according to A. Agassiz (1872, p. 697), 

 to podia with ragged ends. As these cannot exercise an adhesive 

 influence, they may well be sensory in function. 



In the Cassidulidae there is a peculiar series of structures, termed 

 the floscelle, which occurs around the peristome. It is composed of 

 five interradial bourrelets and five radial phyllodes. The bourrelets 

 are formed by the narrowing and thickening of the interambulacra, 

 which form knob-like processes, sometimes slightly overhanging the 

 peristome, and probably homologous in function with the labrum of 

 Spatangids. The phyllodes are expanded, often depressed, leaf-like 

 regions of the ambulacra, and in them there is always some crowding 

 of the pore-pairs. Cotteau (1869, p. 119) states that the size of the 

 phyllodes is in direct proportion to the size of the petals on the 

 adapical surface. This generalization, although in accord with 

 the features of Oolitic forms, is not wholly accurate when applied to 

 Cretaceous and later genera. A plan of the arrangement of the pores 

 in the phyllode of a recent JEchinolampas depressus, Gray, in my 



DECADE V. — VOL. VIII. — NO. VI. 17 



