REPORT ON THE ECHINOIDEA. 27 



the exception of the aberrant genera Heterocidmis, Tetracidaris, and Diplocidaris, 

 which retain more or less Palseechinoid characters while taking on a more modern facies. 



The relationship of the Echinothuridge to the Palaeechinidse I have already insisted 

 upon elsewhere, and their affinities to the recent Diadematidse are most close. The 

 relationship of Hypodiadema to Diademopsis, Pseudodiadema, Hemipedina, and to the 

 whole group of Pseudodiadematidse which culminates in the Chalk, and is only very 

 scantily represented at the present day, is sufficiently near not to need any further 

 elucidation. On the other hand, the development of the Echinidae is somewhat more 

 complicated, as the affinities of the genera from which we can trace the development of 

 the Echinidae, the Arbaciadse, and the Salenidge is very close in the Liasic, the Jurassic, 

 and the Lower Cretaceous beds; where such types as Acrosalenia, Hemicidaris, Glypticus, 

 and Phymechinus, show us how readily we may pass on the one hand to the Salenidse, 

 and on the other to the Temnopleuridse, the Echinidae, and the Arbaciadse. It is, however, 

 only when the interbranching affinities have not extended in too many directions that 

 we can still easily follow the systematic connection, which is as close as we can possibly 

 desire to have it. In fact, it is so extended that we are at a loss to express it satisfactorily. 



A few examples will suffice : from the development of Salenia, of Echinus, of Ternno- 

 pleurus, and of Arhacia, we see that these show a very different degree of complica- 

 tion in their systematic relations to the genera which have preceded them in time. 

 The Scdenice retain the simple ambulacral system of the Cidaridse, the small number 

 of coronal plates, the small number of large primary interambulacral tubercles, the 

 variable shape of the primary spines, the secondary papillae, the large plates of the 

 abactinal system, and, as far as these features in the Cidaridae are related to the 

 Palaeechinidae, the Salenice retain to a less degree the Palseechinid affinities of the 

 Cidaridae. But in addition to this we find in the Scdenice the presence of a subanal 

 plate, comparatively large ambulacral tubercles, a slight tendency in the ambulacral pores 

 to deviate from the vertical arrangement of the Cidaridaj, and in the imbricating plates of 

 the actinal membrane an apparently very decidedly difi"erent structural feature. These 

 last-named features are all features which tend towards the Echinidae proper, and which 

 thus far have not appeared in the older Cidaridae, though we find some of these characters 

 already foreshadowed in the imbricating membrane of the Archaeocidaridge, and in the 

 large primary ambulacral tubercles of the Hemicidaridae. As far as Echinus is concerned, 

 the want of prominence of the principal primary tubercles, as well as the greater uniformity 

 in the structure of the spines, recalls again the earliest Palaeechinidae, while the modifica- 

 tions of the ambulacral system also to a certain extent point back to an ambulacral 

 system made up of a large number of plates, as we find most markedly shown again in 

 the more recent Echinometradae. The actinal membrane is further altered in the direc- 

 tion of that of Scde?iia ; we have a smaller number of plates, which in some genera are 

 reduced to ten, the supports of the buccal tentacles, which are the only remnants of the 



