Vol. 56.] AND OLDHAMIA. 277 



more marked than the complete absence of ordinary ' fossils ' from 

 the slates of Bray Head. 



It still remains possible, however, that some other organism, 

 possibly an annelid, which might easily disappear without leaving 

 other traces of its existence, was responsible for these markings ; 

 and having had occasion recently to consult a memoir by Nathorst 

 (14) on the tracks made by various organisms on the sea-floor, my 

 attention was arrested by two figures representing the tracks of 

 a worm and closely resembling the specimen shown to me by 

 Prof. Watts in the collection of the Irish Geological Survey. 

 This specimen I believe to be an example of the new species 

 Ichnium Wattsii, the type of which is represented in PI. XVII, 

 fig. 1, reproduced from a photograph of a specimen found by my 

 friend and former pupil, Dr. N". Alcock, while we were examining 

 together the rocks at Bray Head. It occurs on the surface of a fine- 

 grained red shale or phyllite : one half, showing the impression in 

 intaglio, is preserved in the Geological Museum of Trinity College, 

 Dublin; the other, in which it is in relief, is in the University 

 Museum, Oxford. On comparing Ichnium Wattsii with the figure 

 (PI. XVII, fig. 4) given by Nathorst of the impression made by a 

 recent worm, Glycera alba or Gonidia maculata, both of which live 

 on muddy bottoms at a depth of 15 feet, a close resemblance will be 

 recognized. The radiating depressions are similarly related to the 

 central area in each ; in neither case do they unite into a single 

 central depression, but plunge downwards independently ; in both 

 they are fairly well defined ; and finally, in both they cross each 

 other and ]ose themselves at the periphery in a manner strictly 

 ■comparable. Thus Ichnium Wattsii does not present a single 

 feature by which it can be distinguished from the track of a 

 recent worm. 



The question next arises as to the relation of Ichnium to Oldhamia. 

 On first writing this paper I assigned the new species to Oldhamia, 

 and though suggesting that it might represent a new genus, I 

 refrained from expressing this in our nomenclature, out of a desire 

 not to unnecessarily add to its burden. But in this parsimony of 

 new terms there was involved unconsciously an economy of logic ; 

 for if Ichnium be Oldhamia, then cadit qiioestio, and Oldhamia is 

 the trace of a worm, but this is a conclusion that cannot be so 

 summarily attained. It becomes necessary to make a further 

 investigation of Oldhamia, and this reveals so many important 

 points of difference between that genus and Ichnium that th< 

 •question of the origin of Oldhamia must be discussed on its own 

 merits. 



The form and character of both species of Oldhamia are far mon 

 definite than is usually supposed, and it is important that the; 

 should be exactly studied, especially as this may lead to theij 

 successful use as zone-fossils. Before proceeding to this task, it ma 

 be convenient to dispose of the arguments of those who hav< 

 attributed an inorganic origin to Oldhamia. In this connexion it is 



