554 Dr. J. R. Kinahan on the Genus Oldhamia. 



given at page 553, taken near the Periwinkle Rocks, Bray, shows, the beds being 

 constituted of a matted mass of polypidoms. These are in layers, compressed, 

 and more or less intermingled with the substance of the rock, which varies 

 from a fine sandstone of extreme hardness and crystalline structure to a friable 

 shaly bed, with but little lamination. The best marked specimens and thickest 

 beds occur in the harder rocks. 



The beds in which Old. antiqua occur are never of the extreme hardness of 

 those in which 0. radiata is found most abundantly. The beds are indiffe- 

 rently either red or green in colour. In the slialy beds, which, much softer in 

 texture, underlie the grit beds, the fossils occur only as scattered fans, or 

 broken and fragmentary filaments. In the denser beds the whole mass is so 

 compressed and intermingled as to render it impossible to make out more than 

 a confused radiating structure. Fig. 5 (p. 552) represents a specimen from such a 

 bed, in which, however, the fossil is better marked than usual ; the section at 

 the right-hand side of the plate shows the true thickness of the bed. Distor- 

 tion by means of cleavage occurs ; this is represented in Fig. 9 ; the woodcut, 

 however, gives but an imperfect idea of the specimen. 



Old. antiqua does not, as a rule, occur in either as dense or thick masses 

 as the other species, and in many stations in Bray it merely, as it were, coats 

 the top of the beds, and even in the thickest series of beds it is seldom, if ever, 

 so abundant as regards specimens ; the beds in which it occurs also readily 

 separate into laminre. At Howth it is equally sparingly diffused in the slaty 

 beds in which it occurs at Puck's Rocks. 



These facts would lead to the surmise that these were animals dwelling in a 

 sandy sea bottom, mud being obnoxious to their existence. 



The species occur distinct, but beds of Old. antiqua are sometimes inter- 

 stratified between those of Old. radiata. One specimen, a quarter of an inch 

 thick, Fig. 3, upper surface, exhibits Old. antiqua with an odd star of Old. ra- 

 diata on one side, and Old. radiata on the other. Scattered fans of Old. anti- 

 qua sometimes occur, though rarely, in the beds of Old. radiata. So that it is 

 evident that the species lived under different conditions, Oldhamia antiqua in 

 the muddier seas, and least abundantly, and that hence the species have been 

 preserved distinct, exactly as at the present day, when two species of animals, 

 inhabitants of the same bay (but either dwelling in different depths, and, there- 



