126 THE VOYAGE OF H.M.S. CHALLENGER. 



same relative increase in the size of the actinal plates of the odd ambulacrum adjoining 

 the actiilostome in all the Spatangoid genera in which there is a tendency to form a more 

 or less distinct actinal groove. In genera allied to the Pourtalesise such as Cysteckinus 

 and Urechinus, there is a similar increase in the size of the ambulacral plates round the 

 actinostome (see PL XXIX." figs. 15-18; PL XXIX." figs. 2/5, 6; PL XXX. fig. 14), 

 but in these genera the actinostome is not vertical but only very slightly sunken below 

 the general level of the actinal surface. 



The whole group of Sj)atangoids to which Pourtalesia, Echinocrepis, CystecJwi^is, 

 Urechinus, Calymne, &c., belong, are remarkable for the large size of the plates of the 

 ambulacral areas (see Pis. XXII.^ XXVII. , XXIX.^ XXX., XXXV.^) compared to the size 

 of the corresponding interambulacral plates. 



In Pourtalesia, Cystechinus, Echinocrepis and the genera above mentioned there are 

 nearly as many ambulacral as interambulacral plates, and with the exception of a little 

 crowding towards the apical system and at the actinostome, the coronal plates of the two 

 areas alternate nearly as regularly as if they belonged to the same system (see PL XXII." 

 figs. 7; 9 ; PL XXVII. fig. 7 ; PL XXIX.^ XXIX.* figs. 1-4 ; PL XXX., XXXV." figs. 

 9-12, and specially the portions of the ambulacral plates with adjoining interambulacral 

 plate of Plate XXIX." fig. '7 ; PL XXVIII.^ fig. 14 ; PL XXXV.^ fig. 8). 



In all the Petalosticha with petaloid ambulacra, the number of ambulacral plates is 

 large in the petaloid portion of the ambulacra, and although it decreases, and the plates 

 become larger from the extremity of the petals towards the actinostome, especially on tlie 

 actinal surface and mainly in the posterior lateral ambulacra, yet they decrease again 

 rapidly when adjoining the actinostome, where two or three or even four ambulacral plates 

 often correspond to a single interambulacral plate of the adjoining area. 



In the Clypeastroids the same thing takes place in aU those in which we have petaloid 

 ambulacra, while it is in such genera as Galerites, Discoidea, Hyhoclypus which have not 

 petaloid ambulacra proper that the ambulacral plates are more uniform in size. 



It is in the Ananchytidse, Dysasteridfe, and the like among the Petalosticha that this 

 increase in the size of the ambulacral plates takes place, and it is also among the Dysas- 

 teridse that we find, as in the modern genera allied to Cystechinus, the elliptical actinostome 

 only slightly sunken below the level of the actinal region. 



All these modern Echinids, however much they resemble the Ananchytidse and Dysas- 

 teridse in general a23pearance, and in the structure of theu- apical system, yet difier 

 radically in having only simple pores for the passage of the ambulacral suckers, piercing- 

 each plate near the centre, extending from the abactinal region to the few plates with 

 double pores near the actinostome which carry the so-caUed gills of the Spatangoids. 



In the Petalosticha it is also among the Dysasteridse that the sunken anal groove 

 makes its appearance in such genera as Metaporhinus,, and among the Ananchytidse in 

 Cardiaster. 



