1881.] PROF. F. J. BELL ON THE ECHINOMETRID^. 



COLOBOCENTROTUS ATRATUS \ 



421 



CoLOBOCENTROTUS MERTENSI. 



ECHINOMETRA. 



The characters of the different parts of the buccal apparatus seem 

 here, as in so many other genera of regular Echinoidea, to present just 

 those slight differences in detail which are so important an aid in the 

 accurate discrimination of species. 



The alveolar foramen, never large, is larger in E. vanbrunti and 

 E. viridis (where it is nearly half as long as the whole alveolus), than 

 it is in E. lucunter or E. subangularis (where it is very distinctly 

 less than half the length) ; it is smallest in E. lucunter. 



The radius is simplest in E. lucunter, widening only very gra- 

 dually and very slowly, and not having its free end notched ; in E. 

 vanbrunti it is a little longer, distinctly wider, but only faintly 

 notched. In E. viridis and E. subangularis the free end is wider ; 

 and in E. subangularis it is hammer-shaped, owing to its somewhat 

 sudden widening out at its free end ; but there is only a feebly deve- 

 loped notch. In E. viridis the notch is more distinct than in any of 

 the three just mentioned species. 



When the observer looks straight through the alveolar foramen, 

 holding the tooth vertically, a delicate ascending and descending 

 process on either side is to be observed in E. vanbrunti ; in E. 

 lucunter the ascending process can just be detected ; in E. subangu- 

 laris neither process can be seen ; while in E. viridis it is the 

 descending, instead of the ascending, process which is visible. 



' This species would seem to be figured in the Phil. Trans, vol. xlix. (1755), 

 pi. yiii. flg. 3. 



