256 



111 regard to the Australian re,presentatives of the genus, I 

 remark that the inner margins of the septa for some distance 

 in the fossa are free, and not fused with the papilli. This 

 is plainly seen in recent examples, as well as in well preserved 

 fossil ones. In a transverse section of a corallum the columella 

 below its papillose surface presents a fascicular appearance, 

 which is due no doubt to its having been formed by the fusion 

 across the axial space of successive growths from the septal 

 margins. In vertical section across the shorter diameter of 

 the coral the columella shows as an irregular j)illar, with 

 lateral nodular offshoots to the adjoining septa on either sich . 

 A columella constructed on such a plan may, I think, be termed 

 essential. 



The species from Alabama described by Edwards and Haime 

 have two kinds of costse. Those on the middle of the broad 

 surfaces of the corallum enlarge towards the calice, and those 

 on the edges near the base are extended and large. A devia- 

 tion from the type species in regard to both the costae and septa 

 was admitted by de Gregorio, who says concerning P. (lai- 

 bornensis : — "This species differs from the two preceding {1\ 

 Stokesii and F. 'Goldfussi) on account of its much more nume- 

 rous, narrower, and less regular cost« and septa."* Now, the 

 Australian species show further that the contrast alluded to by 

 Edwards and Haime between the costae on the edges and those 

 on the flat surfaces cannot be regarded as of generic value. 

 Of the four species of Platytrochus here described, one {P. 

 vacuus, Ten. Woods), exemplifies the Alabama type of costae 

 (pi. v., fig. 4a), but the three others either have the 

 costae of the edges and sides much alike, or the distinction is 

 trifling. Still, the calicular iplan is the same, and in other 

 respects also they are so closely allied to that species that they 

 must be included with it in the genus. 



In all, six species of Platytrochus are known in Australia, 

 viz., one both fossil and recent, four fossil only, and one recent 

 only. 



Platytpoehus Airensis, s/^ec. nov. 

 PI. v., figs. Iff, h. 



Corallum free, compressed, and wedge-shaped, with the edges 

 slightly sloping from the calice downwards, but contracting sud- 

 denly near the base, and then tapering to a point (in the 

 corallum figured the extreme point is broken off). Calice 

 shallow and elliptical, with its major and minor axes in the 



* Loc. eit. 



