286 Proceedings of the Boyal Physical Society. 



importance, from a typical point of view, are the specimens 

 used by Professor Morris, and forming a part of the Strze- 

 lecki Collection, now in the British Museum. In the 

 absence of Mr G. Sowerby's type these specimens must be 

 so accepted, and to their structure all future appeals must be 

 made in determining the identity of P. Irachythcerus (G. Sow.), 

 Morris. Now, as stated above, Professor De Koninck regards 

 the silicified cast* with the long septum, described by Morris 

 as P. Irachythcerus, to be specifically distinct from the true 

 P. hrachythcerus^f a name which he considers should be 

 retained for a form with a short septum, amongst other 

 characters. Now the silicified cast figured by Morris in 

 Strzelecki's work (PL XIV., f. 4a and h) has for one of its most 

 distinctive characters a long septum in the dorsal valve, as 

 previously pointed out, and on looking this matter up it 

 became a question with Dr H. Woodward and myself 

 whether the view advanced by De Koninck was not the 

 correct one, especially as the example of P. Irachythcerus 

 represented by the sandstone cast accorded much better with 

 Sowerby's description of his species, than did the silicious 

 example. The specimen in question,]: as then exposed, was 

 that of a ventral valve backed up with matrix, so that it 

 became a nice point, whether, on the removal of this, in itself 

 a very delicate operation with so fragile a specimen, the cast 

 of the dorsal valve would be visible and the septum exposed, 

 long or short as the case might be. After due consideration 

 we determined that an effort should be made, and the speci- 

 men was accordingly handed over to Mr Barlow, the Depart- 

 mental Lapidary at the British Museum, for manipulation. 

 Extreme care enabled Mr Barlow to remove the whole of the 

 matrix without very much damage to the specimen, consider- 

 ing the delicacy of the operation, and with this result, that it 

 was found to possess, so far as we are able to judge, a short 

 septum, so probably confirming in a remarkable manner 

 Professor De Koninck's surmise. The matter then stands 

 thus — the figure given by Morris in Strzelecki's work on 

 " New South Wales," PI. XIV., f. 4c, is the true P. Irachythcerus, 



* Strzelecki, pi. 14, f. 4a and b. + Loc. city f. 4« and h. 



X strzelecki, loc. ciL, jpl. 14, f. 4c. 



