TRANSACTIONS OF THE SECTIONS. 83 
Throughout all these gradations of form, however, it must be borne in mind that 
the minute structure of the parts is such as at once to enable the observer to di- 
stinguish Cephalaspis from Pteraspis. 
I ventured in my paper read before the Geological Society, to say that the Ganoid 
nature of Cephalaspis and Pteraspis appeared to me to be unproved, and to allude to 
the relations between these genera and the great group of existing Siluroideit. With- 
out at all wishing to push too far resemblances which, when we come to know more, 
may turn out to be mere analogies, I must say that the new facts which I have brought 
forward appear to point somewhat in the same direction. The siluroid fishes, in fact, 
are especially characterized by the large bony nuchal plate which supports the great 
dorsal spine, and is constantly anchylosed to the posterior margin of the skull. The 
rostrum of the Péeraspis is not without its analogue in Loricaria. 
On the other hand, that rostrum might be compared with the prolonged snout of 
Acipenser; and the shield of Pteraspis presents many points of similarity with the 
cephalic buckler of such fishes as Dipterus, Diplopterus, and Osteolepis. 
On the Jointed Structure of Rocks, particularly as developed in several places 
in Ireland. By Professor Witt1aAM Kine, F.G.S., Queen’s College, 
Galway. 
The author, adverting in the first place to the view which he advanced at the Dub- 
lin Meeting on the so-called slaty cleavage in its perfect state, that it is the com- 
bined effect of jointing, and of pressure exerted more or less perpendicularly to its 
planes, stated, that he had since visited several localities in Mayo, Galway, Clare, 
Limerick, Tipperary, Kerry, and Cork, and had obtained a number of important data 
fully bearing out his view. His researches had also afforded him much insight into 
jointing—a divisional structure of more importance in Physical Geology, he thinks, 
than appears to be generally understood. The author then proceeded to describe 
jointing as it occurs in the Carboniferous limestones of Galway and Clare. In these 
counties there may be made out four distinct sets of joints, two of which, from being 
east or west (the number of degrees varying) of compass north, he terms—one, east 
meridional, and the other, west meridional; while the remaining two, which more or 
less rectangle the former, he designates—one, east equatorial, and the other, south 
equatorial. The east meridional set is the strongest and most regular: it runs in 
an undeviating course for miles and miles, generally from N.N.E. to 8.S.W.; plains, 
valleys and mountains being all alike affected by it. In many places it cuts the 
limestone into plates from half an inch to little more than an inch in thickness; a 
circumstance, favouring the idea, that, under proper conditions, jointing may be 
developed to the utmost possible extent. None of the other three sets is so well 
developed ; the south equatorial, which runs a few degrees south of west, is with 
some difficulty made out. In consideration of the various phenomena which occurred 
to him, he has formed the conclusion, that only horizontal or undisturbed beds ex- 
hibit the original or normal direction of the joints with any certainty ; and consider- 
ing that the beds which have the east meridional set running N.N.E. and 8.S8.W. 
are horizontal, he concludes that such is the normal bearing of this set; and he fur- 
ther assumes, that any deviation from such direction has been caused by disturbances 
of the strata. Entertaining these views, he applies them in elucidating and reconciling 
certain complicated and seeming discordant phenomena; showing that some highly 
inclined beds, striking east and west, like the limestones of Cork, may have one of 
their sets of equatorial jointing lying horizontally ; while in others, having only their 
basset edges exposed on the surface, the vertical and not the horizontal direction of 
the joints is exhibited. Assuming the principal jointing in Yorkshire, Derbyshire, 
Devonshire, and other counties to run generally from N.N.W. to §.8.E., as stated 
by Phillips, Hopkins, De la Beche and others, he regards the west meridional as the 
dominant set in England. ‘This difference in direction between English and Irish 
jointing is analogous to what occasionally obtains in the Dingle peninsula; and a dif- 
ference still more striking occurs within a space of a few yards or so, in a (Silurian) 
bed on the shore of Lough Mask. In connexion with the close approximation of 
the joints, as rather frequently displayed in the Carboniferous limestone of the West 
of Ireland, attention was directed to that form of divisional structure in coal, known 
6* 
