2o6 THE GENUS MONTICULIPORA. 



round which they are disposed in a single row. The circle 

 thus formed is, however, rarely or never quite complete, and 

 each of the large tubes usually comes into contact at different 

 points with one, two, three, or even four of its neighbours. 

 The small corallites are angular in shape, mostly oblong or 

 trapezoidal, and are very variable in size, being always very 

 much inferior in size to the larger corallites. Vertical sections 

 (fig. 43, b) show also that their internal structure differs entirely 

 from that of the large tubes, and resembles that of the smaller 

 corallites among the Monticidiporidce generally, the visceral 

 chamber being simply crossed by crowded, complete, and 

 horizontal tabulse. 



It is only necessary to compare P. Graya: with P. Nezv- 

 berryi, Nich., and P. Sclwynii, Nich., from both of which it is 

 readily distinguished. From P. Nciubcrryi, Nich., it is separ- 

 ated by the different form of the corallum (which is never leaf- 

 like), by the resulting length of the corallites, and by the pris- 

 matic form of the large tubes, as well as by other structural 

 features. From P. Selzuynii, Nich., the present form is dis- 

 tinguished by the prismatic, not oval or circular, form of the 

 large tubes, by the want of spiniform corallites, by the fact 

 that the tabulse of the large tubes are not cxccntrically incom- 

 plete, and by the fact that the small tubes are not aggregated 

 into clusters. 



Horizon and Locality. — Abundant in the " Craighead Lime- 

 stone" (Lower Silurian), Girvan, Ayrshire. 



Monticulipora (Prasopora) Selwynii, Nich. 

 (Figs. 44 and 45.) 



Spec. Char. — Corallum (in the typical examples from the 

 Trenton Limestone of Canada) of comparatively large size, 

 varying from an inch and three-quarters up to three inches 

 and a half in diameter, the height in either case being about 

 an inch in the centre, hi form the corallum is discoid, or 



