Vol. 56.] AND OLDHAMIA. 279 



most weighty arguments against the organic origin of OWiamia 

 have been considered, and, it is hoped, disposed of, we may proceed 

 to a description of the two Irish species of the genus, the result of 

 a prolonged and careful investigation under the microscope. 



Oldhamia bajdiata. (Pis. XVII-XIX.) 



This is a radiate system of branching grooves, proceeding from a 

 central area (PI. XVII, fig. 2). The central area may be depressed 

 below the general level of the surface of the phyllite, or raised tent- 

 like above it, more frequently the latter ; in some cases it is occupied 

 by the origin of the branches, in others marked by more or less 

 hemispherical pits ; occasionally it is a plain surface, and some- 

 times, but not always, defined by a faintly-impressed closed curve. 

 The radiate grooves are very sharply defined, variable in number, 

 narrow, about 0*4 mm. in breadth, repeatedly branching till they 

 terminate in club-shaped processes. These processes are narrow at 

 the origin, but gradually grow broader till they attain the breadth 

 of the main grooves, and terminate in a rounded extremity ; their 

 length, though variable, is as a rule about 3 mm. Not infre- 

 quently the terminal processes fail to show continuity with the 

 rest of the impression : this is, in some cases, the result of the 

 excessive attenuation of the proximal extremity ; in others the dis- 

 continuity is absolute, and no connexion between the process and 

 the chief grooves can be discovered. Usually, but not always, the 

 club-shaped processes are remarkably straight ; frequently they are 

 constricted at more or less regular intervals, so as to present the 

 appearance of a row of hemispherical pits. These pits are pre- 

 sumably the 'cells ' of J. R. Kinahan's description, which are stated 

 by that author to be biserial in arrangement. This I have not yet 

 observed; in my specimens they form but a single series. In 

 some specimens long isolated tenuous furrows, 25 mm. in length, 

 3 mm. in breadth, extend for a considerable distance beyond the 

 general outline of the marking, and may be occasionally seen to ter- 

 minate in a club-shaped extremity, after having given off on one side 

 one or two other club-shaped processes (PL XIX, fig. 13) : there is 

 no discoverable constriction at the origin of these processes. These 

 isolated sprays of the main impression are particularly instructive, 

 since as a rule the crowding of the branches renders it difficult to 

 ascertain their true characters. Occasionally disconnected, simple, 

 straight, semicylindrical grooves, and hemispherical pits lie scattered 

 on the surface of the phyllite in the vicinity of a radiate system. The 

 phyllites in which Oldhamia occurs are generally devoid of all traces 

 of transverse cleavage ; but the effects of tangential pressure are 

 shown in many instances by a compression of those grooves of a 

 system, which extend at right angles to what must have been its 

 direction, and I was thus led to suspect that the constrictions which 

 give a beaded appearance to the club-shaped processes might also 

 have resulted from its action. This, however, is plainly not the 



