ECHINIDAE. 459 



as is desirable. The position of such genera as Mespilia and Amblypneustes, 

 and of the recent representatives of Pseudodiadema and its allies, must remain 

 very doubtful. 



The subfamilies of Triplechinidae and of Temnopleuridae, into which the 

 Echinidae are divided, both contain representatives with the above-mentioned 

 different modes of growth of the poriferous zone. The function of the 

 peculiar pits in the Temnopleuridae at the angles of the plates in both the 

 areas, reduced to mere pores in some of the genera, is not known. This 

 character in Amblypneustes is reduced to its simplest expression, and the 

 sculpture so exaggerated in Temnopleurus and Microcyphus, already re- 

 duced materially in Salmacis, disappears entirely in Amblypneustes proper, 

 though the general features of the genus recall strongly Salmacis, Temno- 

 pleurus, and allied genera, in the structure of the abactinal and actinal sys- 

 tems, while the poriferous zone and the general arrangement of the coronal 

 tubercles are more allied to the Triplechinidae proper. In the second sub- 

 family (Triplechinidae), the arrangement of the poriferous zone in short 

 straight arcs of three pairs is the prominent feature, combined with a 

 more or less sporadic and irregular distribution of the tubercles in the inter- 

 ambulacral area, although in some genera of this subfamily, as in Hemi- 

 pedina, Phymosoina, the coronal plates of the ambulacra! system are so large 

 that the plates of the poriferous zone form a single vertical line of pores. 



More abundant materials than are now accessible, especially in the Temno- 

 pleuridae, will undoubtedly greatly modify the views regarding the species 

 and affinities of these Echini, which are here given as an approximation 

 merely ; for nowhere among the regular Echini do we find such remarkable 

 changes due to growth as in this subfamily, to judge from the few species of 

 which I have had occasion to examine good series of specimens. 



