130 THE, VOYAGE OF H.M.S. CHALLENGER. 



ambulacra the simple ambulacral pores characteristic of the genera of the Pourtalesise proper, 

 such as Pourtalesia, Echinocrepis, Cystechinus, Urechinus, and Calymne, differ from these 

 in having either a labiate actinostome similiar to that of the normal Spatangoids, or in 

 having a lateral fasciole in addition to the more or less rudimentary sulianal fasciole found 

 in the latter group. Tbis second group also shows a tendency in the abactinal region to 

 form rudimentary petals, but in these the tentacles, while passing through paii's of pores, 

 still retain the simple character found in the Pourtalesise proper. 



In Aerope and Aceste the odd anterior ambulacrum takes a great development, the 

 suckers of that area occuioying in one genus the greater part of the abactinal surface 

 within the peripetalous fasciole ; in the other the whole of the same space in the sloping 

 anterior part of the test. This group of Spatangoids brings out the aiiinities of the Pour- 

 talesiee to such Spatangoids as Brissopsis, Hemiaster, Echinocardium, Lavenia, and the 

 like. 



The group of Spatangoids to which Argopatagus, Homolampas, and Paleopmeiistes 

 belong, although it has the highly laljLate actinostome so characteristic of the recent 

 Spatangoids, is characterised by the rudimentary nature of the ambulacra and by the 

 embryonic structure of the ambulacral and interambulacral plates, that is to say, by their 

 uniformity in size. This is specially the case in Argopatagus and Genicopatagus, some- 

 what less in Paleopneustes, while in Homolampas the great difference in the size of the 

 plates composing the ambulacra and interambulacra brings this genus nearer the typical 

 Spatangoid in structure ; in Paleopneustes, the closer relationship is indicated by 

 the tendency to form more or less rudimentary petaloid ambulacra towards the apical 

 system. 



Aerope and Aceste, together with Cionohrissus, form a group of Spatangoids illustra- 

 ting the affinities of the family Pourtalesise with the Brissina. In Aerope and Aceste 

 the actinal surface of the test, while having the normal Spatangoid structure of a more or 

 less weU-developed actinal plastron, yet retains somewhat the cylindrical form of the 

 Pourtalesise and the simple circular actinostome of this family. The ambulacral system, 

 although more simple than in Cionohrissus, is surrounded by a more rudimentary, less 

 distinctly defined peripetalous fasciole ; and we have no trace of any anal snout which is 

 still a prominent feature in Cionohrissus, and very rudimentary in tlie Brissina proper. 

 The sunken odd ambulacrum of Aceste, the great development of the suckers of this 

 ambulacrum in both Aerope and Aceste is a feature with which we had l)ecome familiar 

 in young Brissina. The presence of a large anal fasciole surrounding the posterior part 

 of the test is like'^ise a feature characteristic of the young of that family. In Ciono- 

 brissiis the petaloid system is as greatly developed as in such genera as Brissopsis and Mac- 

 ropneustes. The affinities developed in so many directions in the group of Pourtalesise is 

 one of its most interesting features ; and as we have iust traced the relationship of the 

 Pourtalesise to the Brissina, and to such genera as Hemiaster, Echinocardium, Lovenia, 



