DESCRIPTIONS OF GENERA AND SPECIES 



631 



They are of various sizes: Pontoscolex being one of the smallest and Geoscolex the 

 largest forms. The prostomium is not always present ; this being nearly the only group 

 of Oligochaeta in which this characteristically Chaetopocl organ is occasionally deficient ; 

 the only other instance is Deodriius. Those genera in which the prostomium is thus 

 absent form a natural section of the subfamily ; ROSA, indeed (20) puts these three 



N.B. It must be explained that the thick lines indicate paired caeca ; the 

 thin lines only slightly differentiated oesophagus. It is not, however, possible 

 to institute a very inflexible demarcation between the one and the other. 



genera, Pontoscolex, Diachaeta, and Onychochaeta into a single genus ; I am not, 

 however, inclined to follow him so far. 



These three genera have also an irregular arrangement of the setae, at any rate 

 in the posterior region of the body ; if this did not also occur in T.rwhochaeta, a genus 

 which possesses a long prostomium like that of Rhinodrilus, it would be, perhaps, 



