19^7-] J. vStephenson : Indian Oligochaeta. 363 



Octochaetinae begin with Howascolex ; they then retain the cha- 

 racter of a monophyletic and compact subdivision of the family 

 If however we derive Hopiochaetella from Perieodrilus we have 

 either to include Hopiochaetella and Erythraeodrilus in 'the Acln- 

 thodrilmae or to make a separate subfamily for them As 

 Michaelsen has pomted out (13) - the family Megascolecidae is a 

 much-branched tree, which took its origin from the acanthodri ne 

 primordial form, that is to say, from Notiodrilus [Eodrilus) The 

 greater branches of this tree, the different subfamilies, are well de- 

 fined in their distal parts." The original form, wi{h the small 

 branches clustering round the base of the tree, constitute the group 

 Acanthodrilinae; on the present supposition one of these small 

 branches scarcely large enough to form a separate subfamilv con- 

 sists of Peneodnliis—Hoplochaetella—Erythraeodrilus 



The finding of additional intermediate forms' between the 

 various genera will establish the lines of filiation more certainly. 

 The Megascolecidae are an especially favourable group for the work- 

 ing out of phylogenetic relations, because we still possess what we 

 might paradoxically call a living Palaeontology. We can for ex 

 ample, trace the main line of descent of the Megascolecinae from 

 the original Acanthodriline to Pheretima, with hardly a break by 

 means of actually existing genera ; it is as if Phenacodus and 'the 

 subsequent stages in the phylogeny of the Horse were all alive to- 

 day. In giving rise to descendants, the older genera have them- 

 selves lived on, scarcely modified ; and what we have to hope for 

 is the discovery of the still missing intermediate forms. Some of 

 tnem, at least, we may reasonably expect to find, still alive and 

 accessible to a complete investigation. 



The two following schemes represent the alternative possibili- 

 ties regarding the position and descent of Hopiochaetella and 

 trvthraeodnlus : — 



\ . / 



— Nottodnlus — 



/ I ^Plagiochaeta 



Diplotrema \ Perieodrilus 



I Howascolex 



to Megascolecinae / \ 



Hopiochaetella Octochaetus 

 Erythraeodrilus Eutyphoeus Dinodrilus 



\ . . / 



— N otiodrilus — 



„.,- / \ Plagiochaeta 



Diplotrema Perieodrilus 



Hoi&ascolex 1 



/ Hopiochaetella 



Octochaetus \ 



/ \ Erythraeodrilus 



Eutyphoeus Dinodrilus 



