108 



cimens in having the ambulacra distinctly wider than the interambulacra, so they 

 must be referred to this si)ecies, according to our present knowledge. I should 

 not, however, be very surprised, if there sliould prove to exist more than one 

 species of Holopneustes with such wide ambulacra. 



I give here some measurements of the specimens before me of the three 

 Holopneiistes-apecies. These give very good distinguishing characters, which are the 

 more needed as no specific characters are found in pedicellariæ, spicules or structure 

 of spines. 



Holopn. pui'purascens 



- (?) 



— inHatus 



porosissimus 



(Tyjïe-specimen.) 



(The measurements oi'intorporifertuis and pori- 

 ferous zones logethergivc 1.3 mm ; the measure- 

 ment 12 mm. for the whole ambuUicral area is 

 due partly to the roundinfi of the test, partly 

 to inevitable small errors in the measuring.) 



(Type-specimen.) 



These researches on the Amblijpneustes- and Holopneustes-species seem to 

 show that they are not so difficult to distinguish as commonly believed, when only 

 the specimens are preserved with Iheir spines etc., whereas naked tests cannot al- 

 ways be distinguished with full certainty, at any rate in the present state of our 

 knowledge. The analytical table of these species given below will, I hope, prove 

 to be useful. (Due allowance, however, must be made for the uncertainty, whether 

 the species named A. pallidiis, is really that species; comp, above p. 105.) In this 

 table I also include the species of Microcyphus, so nuich like Amblypneustes (^except 

 M. maculatiis), and Goniopneiistes. Upon the whole these species differ very little 

 in appearance and will certainly be confounded without a close examination, as 

 I have shown to have been the case in several instances. 



