550 



Dr. J. E. KiNAHAN on the Genus Oldhamia. 



arrived at this conclusion as to their nature, long before I read Foebes's opi- 

 nion on the subject, or before I was aware of its existence, as my study of 

 the fossil was made in the field in the first instance as a mere amusement, not 

 ■with the object of either establishing or overturning any theories. I feel 

 strengthened in my opinion, as I find that every naturalist I have spoken to 

 on the subject, or shown my specimens to, since, seemed to be of the same opi- 

 nion. It must, at the same time, be confessed, that to bring forward any argu- 

 ment in favour of Hydrozoa over Polyzoa, which will appear conclusive on 

 paper, is a task of extreme difficulty. The distinctions between the two alli- 

 ances, though tangible enough to the practised eye, cannot be described in 

 such precise language as will be diagnostic to every reader; these distinctions 

 being only appreciable on a careful and patient comparison of the general cha- 

 racters of an obscure structure, which, owing to the similar mode of growth in 

 the two groups of animals, is almost undistinguishable in the two cases ; and by 

 a cautious consideration of the true bearing on the general character of the 

 two groups, of those several points of structure which are yet appreciable. 



This difficulty, need not cause any astonishment, when it is recollected that 

 but a few years since these two great groups, the Polyzoan Mollusc and the 

 Hydrozoan Acrite, were unhesitatingly classed together; and furthermore, that 

 even among recent species, widely different as the animals are in their structure, 

 and the relation of their organs, yet in many cases, so similar 

 is the arrangement of their parts, it is impossible to decide 

 a priori from the skeleton whether it is a Polyzoan or 

 Sertularian Hydrozoan which is under examination, — in 

 our present state of knowledge, a reference to the soft 

 parts alone enabling us positively to determine the ques- 

 tion. 



If we compare Oldhamia antiqua (Fig. 1) with speci- 

 mens of Sertularia argentea, the resemblance is so close, 

 that some years since these would have been by palaeonto- 

 logists pronounced specifically identical. In fact, by embed- 

 ding in plaster the dried polypidom oiS. argentea dredged at 

 Bray, and from this artificial fossil taking a cast, I have ob- 

 tained casts which, in no respect, differ from the specimen 

 of Oldhamia, of which Fig. 1 is an engraving. 



Fig. 1. Oldhamia anti- 

 qua: — erect polypidom ex- 

 hibiting alternate mode of 

 growth of fasciculi of 

 branchlets and ordinary 

 appearance of fossil. Be- 

 yond Brandy Hole, Bray 

 Head. 



