Prof. S. Lov(5n on the Structure of the Echinoidea. 383 



Palceostoma, Grray, at least in young individuals ; I have no 

 full-grown ones at my disposal. In the whole of this group 

 consequently the arrangement, especially in the interradia, is 

 much less strict and symmetrical than in those with infraanal 

 fasciolge. These latter seem to prevail among the recent 

 Spatangida3, but were apparently represented only in the 

 genus Micraster during the Cretaceous period, the other 

 genera of which either want .the fasciola or have it peripetalous 

 or composite, and with this, we may assume, a less regular 

 arrangement of the plates. 



A strongly depressed form of body, the proximity of the two 

 surfaces (ventral and dorsal) to each other, their union internally 

 by means of processes, pillars, arches, and chambers, the dis- 

 tribution of the very numerous tentacular pores even upon the 

 interradia, the madreporite, which, in most, occupies all the five 

 apical plates, the position of the genital pores not always in the 

 apical plates, but separate from them in the interradia — all 

 these ai*e characters which, with others, distinguish the Cly- 

 peastridge from the other irregular Echinoidea. On another 

 side they approach the regular Echinoidea by the presence of 

 jaws, by the small alterability in form of the peristome during 

 growth, which is dependent upon this, and its central position 

 in the ventral surface, opposite to the pentagon of the vertical 

 and eye-plates in the dorsal surface, in which only the abnormal 

 Dendraster and some few others present any deviation. When 

 full-grown, moreover, they have, in many genera, all the five 

 ambulacra alike ; whilst in others the bivium, to a certain ex- 

 tent, becomes apparent early, or gradually, by the movements 

 and changes in the form and size of the plates, which are more 

 considerable here during growth than in other Echinoidea. 

 L. Agassiz and Johannes Miiller observed how, in the Clype- 

 astridee, the corona simplifies itself towards the mouth, how the 

 plates increase more in breadth than in length (as had already 

 been noticed by Philippi in Echinus)^ and how this applies most 

 to the ambulacral plates, which are inserted into each other. 



In the fully developed state Echinocyamus pusillus and 

 Laganum depressum^ both of which have all the five interradia 

 connected in an uninterrupted sequence of plates, are regular, 

 with all the five ambulacra similar, and, with the exception of 

 the periproctium, essentially also all the interradia — as also 

 Encope Valenciennesi and E. Stokesij Clypeaster rosaceus and 

 Stolonoclypihs prostratuSj in which, in all the ambulacra of both 

 bivium and trivium, plate 2 in the former and plates 2 and 3 

 in the latter are so enlarged in breadth that by means of them, 

 in their outer angles, all the ambulacra touch each other, form 

 a complete circle, and shut off plate 2 and the following onet; 



