454 PEOE. p. M. DUNCAN ON THE GENUS PLEUEECHINUS. 



crenulate tubercles, the distinction is of some classificatory im- 

 portance. 



The classificatory position assigned by A. Agassiz to Pleur- 

 echinus in the ' Revision ' must be conceded, and it is a subgenus 

 or section of Temnopleurus. 



It might be contended that Pleurechinus, having non-crenulate 

 tubercules, is identical with TemnecTiinus, Forbes ; and A. Agassiz 

 evidently considers it closely allied to the Nummulitic members 

 of the Temnopleurid group described by D'Archiac and Haime. 



But Temnechinus,'EovhQi,\\Si^ no true pits at thesutural angles, 

 and the depressions over the sutural margins do not therefore 

 terminate in deep inward undermining or penetrations of the 

 test. 



A careful examination of the tests of the Crag Temnechini in 

 the British Museum, the Eoyal School of Mines, and in the 

 collection of the Geological Society, indicates that none of the 

 remarkable minute structures of the test of Temnopleurus are 

 present. 



Forbes's determination of the lack of pits at the angles, de- 

 pressions only existing, and of the absence of crenulation in 

 Tem7iec}iinus\o\diB good ; and no forms can enter the genus which 

 have true pits and crenulated tubercles. 



Pleurechinus is therefore not synonymous with Temnecliinus, 

 and, as may be gleaned from my communication on the Temno- 

 pleuridse to this Society (Proc. Linn, Soc, Zool. 1882, vol. xvi. 

 p. 343), they belong to different genera. 



The Temnopleuridae from Sind, referred to by A. Agassiz, have 

 no true pits ; but the tubercles are crenulate. Their state of 

 preservation and the absence of the apical system prevent their 

 being satisfactorily classified. But amongst the Temnopleuridae 

 lately received from Sind there are forms with a remarkable apical 

 system, and a condition of the outside of the test which brings 

 them under distinct genera. It is probable that the forms 

 described by D'Archiac and Haime will come under one of these 

 generic divisions, so that the whole series will be in alliance with 

 GlyphocypTius (Haime) and Trigonocidaris (Agass.). 



