No. 3.] NICHOLSON — NEW FOSSILS. 145 



.inch to nearly three-quarters. The calice is oblique, so that the 

 srreatest leno-th of the coral is alons; its convex curvature. 



Petraia Logani is closely allied to Petraia (^TurhinoJopsis) 

 ,pluriradialis, Phillips, with which I was at first sight disposed 

 to identify it. It is, however, readily distinguished by the flat- 

 tening of the convex curvature and lateral aspects of the coral, 

 and by the smaller number of radiating septa. As regards other 

 more minute characters, the published descriptions of P. pliiri- 

 radiaUs are not sufficient to enable any closer comparison to be 

 instituted with advantage between the two species. 



There exists also a singular, and in some respects inexplicable, 

 .Tesemblance between the form here described under the name of 

 P. Logani. and that described by Mr. Billings under the name 

 of Heliophyllum exigimm (Can. Journ. New Series, Vol. Y. p. 

 261); at the same time that differences of such gravity exist 

 that the two forms cannot be united under the same specific 

 title, or even placed in the same genus. Without pretending at 

 present to explain the discrepancies of observation here alluded 

 to, it may be as well to present in a summary form the points of 

 agreement and difierence which appear to exist between the twa 

 ; species. 



1 . Both corals are of the same general form and size, and 

 occur not only in the same formation, but also at the same lo- 

 cality. 



2. Both corals are alleged to possess externally a couple of 

 straight septal ridges, extending from the top to the bottom of 

 the coral, and having the other septa directed obliquely towards 

 this line on both sides. I have, however, never been able to de- 

 tect this structure in the comparatively few specimens which 

 have come under my notice, which I should feel disposed to 

 refer to H. exiguum. 



3. The number of septa in the cup appears to be about the 

 same in both, though said to be sometimes as many as eighty in 

 H. exiguwn, whilst they never appear to exceed sixty-five in 

 -P. Logani. 



Whilst the above are the chief points of agreement, there are 

 'the following points of difference to be noted : 



1. H. exiguum, though this is not specially alluded to, must 

 possess more or less well developed tahulce ; but no traces of such 

 structures can be detected in P. Logani, in broken specimens or 

 in longitudinal sections. 

 Vol. YII. k No. 3. 



