REPORT ON THE ECHINOIDEA. 129 



In the normal Spatangoids tlie odd interanibulacnim on the actinal surface consists 

 generally of three large plates, which occupy the whole space between the actinostome and 

 the edge of the ambitus. In the Pourtalesise and the allied genera, Spatagocystis, Echino- 

 crepis, Cystechinus, and Urechinus, this odd interambulacrum is made up of a large 

 number of small plates, none of them having the preponderance in size so marked in the 

 majority of recent Spatangoids and forming the well-marked actiual plastron of such 

 genera as Schizaster, Maretia, Metalia, Hemiaster, &c. It is in genera like Palceotropus 

 and Genicopatagus that the actinal plastron has a somewhat greater prominence till it 

 becomes in such types as Homolampas, Paleopneustes, and Argopatagus nearly as promi- 

 nent as in the Spatangoids proper mentioned above. This character of the absence of 

 a true actinal plastron is characteristic of the Petalostichan genera which retain Clypeas- 

 troid' or Echinolampan features, and we find it, consequently, wanting in such families as 

 the Cassidulida3 and Dysasteridaj, while in the Ananchytidas the actinal plastron is a pro- 

 minent structural feature of the family which gradually becomes more marked in the other 

 families of Spatangoids. 



.The genus Spatagocystis forms the passage l^etween such genera as Holaster, Cardi- 

 aster, Ananchytes, Infulaster, and Pourtalesia. From above it has somewhat the 

 general outline of the pointed form of Pourtalesia lagnncula, while the test has the 

 general appearance of Cardlastcr and Ananchytes combined with the sunken actinostome 

 of Pourtalesia and the presence of a small anal snout, with the slightly sunken anterior 

 ambulacral region of Infulaster; while Echinocrepis forms the passage from the 

 Pourtalesise to such Ananchytid forms as Cystechinus and Urechinus so far as the 

 structure of the anal system is concerned. The test of Spyatagocystis has neither the 

 angular outline of Echinocrepis nor the bottle form of Pourtalesia, but has the swollen 

 rounded contour of Spatangoids like Micraster and Holaster. 



The anal snout formed in Sp>atagocystis resembles more the beak in prolongation of the 

 subanal plastron of some of the Spatangoid genera, such as Echinocardmm and the like. 

 With the overhanging abactinal hood and the small beak of Sjmtagocystis we readily trace 

 the passage to such an anal extremity as that of Urechinus and Echinocrepis, in which the 

 actinal slope forms a more or less bevelled surface with rounded angles to the actinal and 

 abactinal surfaces, on which is situated the more or less sunken anal system. The pro- 

 minence of the anal snout plays an important part in deflecting the odd interambulacrum 

 from its course, or in stopping it even altogether from reaching the actinal surface or dis- 

 connecting it from the actinal plastron. 



In the group of Spatangoids to which we now come, the genera Argopatagus, Homo- 

 lampas, Genicopatagus, Linopneustes, and Paleopneustes, while having for a part of the 



1 The Clypeastroid affinities of the Pourtalesise consist in the simple actinostome and in the structvue of some of the 

 pedicellariae which are like those of the Clypeastroids and Echinolampadte. 



(ZOOL. CHALL. EXP. — PART IX. 1881.) . 1^7 



