REPORT ON THE ECHINOIDEA. 157 



absence of genital pores, it is difficult to analyse this apical system with certainty. The 

 arrangement of the primary tuloercles is much Hke that of Cystechimts ; from two to five 

 primary tubercles, varying considerably in size, occupy the coronal plates both of the 

 ambulacral and interambukcral areas on the abactinal side. The spines vary greatly in 

 shape in different parts of the test (Plate XXXIV. figs, lla-f). The cluster of primary 

 tubercles of the posterior abactinal region of the test above the anal system, carry paddle- 

 shaped radioles (PI. XXXIV. fig. 11); on the anterior part of the test and the ridge 

 extending to the apical system they are more elongate, often sharp (PI. XXXIV. 

 figs. 11«, d). The same diversity is also found on the actinal side, where the paddle- 

 .shaped primary radioles are concentrated on the actinal keel and near the posterior 

 extremity ; the tubercles of other parts of the actinal surface, carrying very differently 

 shaped radioles. The miharies are uniformly scattered over the test, distant, carrying 

 short, slender sharp spines. 



Fayal; 2650 fathoms. May 27, 1873. 



PalcEOtropus. 

 Palceotrojms, 'Lovin, 1874, Etudes surlesEchinoiddes, p. 17 (Kongl. Svensk. Vet. Akad. Handl., vol. xi. No. 7). 



The systematic position of the deep-sea genera allied to Palceotropus in having simple 

 ambulacra, extending from the actinostome to the apical system, suggests for criticism 

 the relationship of all the other Spatangoids when tested by this character alone. The 

 genera, which like Palceoti-ojuis, Pourtcdesia, Echinocrepis, Spatagocystis, Cystechinus, 

 and Urechinus have only simple pores extending from the actinostome to the apical 

 system in all the ambulacra differ from all the other Spatangoids living and fossil 

 (except it be genera hke Infulaster, of which the structure of the ambulacra is not 

 well known). Some of the Spatangoids proper are characterised by the difference in 

 structure of the anterior ambulacrum and the lateral ambulacra ; such genera, for instance, 

 as Brissus, Meoma, Faorina, Desoria, and Brissopsis, in which in the anterior ambula- 

 crum the pores are brought together and extend singly from the apex to the actinostome 

 (except immediately round the actinostome where the pores are separated again). In 

 others as in Hemiaster, PalcBostoma, Schizaster, Spatangus and Plagionotus the pores 

 are separated near the apical system in all the ambulacra within the peripetalous fasciole, 

 while they are so separated within the fasciole only in the lateral ambulacra of the former 

 group, and in both groups they are simple again below the peripetalous fasciole. 



In genera without distinct petals, such as lAnopneustes, Paleopjneustes, Homolamjxis, 

 Argopatagus and Genicopatagus the arrangement of the pores in all the ambulacra 

 closely resembles that of such genera as Collyrites, Ananchytes, and Holaster, while in 

 Micraster we have the first indication of the speciaHsation found in the grouja to which 

 Hemiaster, Spatangus, Paheostoma, and the like belong. 



Among the older genera Hemipneustes, and among the recent genera Agassizia, on 



