Sir P. de M. Grey Egerton on the genus Pleuracanthus. 423 



XLII. — On the Unity of the genera Pleuracanthii^s, Diplodus and 

 Xenacanthus, and on the Spscific Distinction of the Permian 

 Fossil Xenacauthus Decheni {Beyrich). By Sir Philip de 

 Malpas Grey Egerton, Bait., F.R.S. 



To the Editors of the Annals of Natural History. 



Gentlemen, 



I am induced to offer the following notice for publication in 

 tlie Annals of Natural History by the strong feeling I entertain 

 (in common, I believe, with all naturalists) of the advantage of 

 correcting and amending the nomenclature of the various objects 

 of our studies in accordance with the progress of scientific know- 

 ledge and the discoveries of the day. 



The genus Pleuracanthus was established by Professor Agassiz 

 in the year 1837. The only portion of the fish then discovered 

 was the defensive spine, the resemblance of which to the dagger 

 of the Trygon and other armed Rays induced him to classify 

 Pleuracanthus with that family. The species P. Icevissimus was 

 found in the Dudley coal-field. Two other species were sub- 

 sequently named, found in the coal-fields of Leeds and North 

 Wales. More recently these peculiar spines have been found in 

 considerable abundance in the neighbourhood of Edinburgh and 

 at Carluke, and three additional species have been described by 

 Dr. Newberry from the coal-measures of Ohio. In the year 

 1834 I discovered, in the coal-shale of the Silverdale Mine in 

 North Staffordshire, some remarkable tricuspid teeth, to which 

 Agassiz assigned the new generic title Diplodus. Similar teeth 

 were discovered about the same time by the late Dr. Hibbert Ware 

 in the neighbourhood of Edinburgh, and by Mr. Rankin at Car- 

 luke. They have since proved of common occurrence in most 

 of our British coal-fields, and I have also received specimens 

 from Prof. Dawson from the same formation in Nova Scotia. 



In the 'Jahrbuch' for 1849, Professor Beyrich gives a detailed 

 description of Xenacanthus Decheni, a most remarkable fish found 

 in the Rothe-todt-liegende at Riippersdorf in Bohemia. The 

 general form of the fish resembles Squatina, but its most remark- 

 able feature is the insertion of a defensive spine immediately be- 

 hind the occiput. This spine is described as having the greatest 

 resemblance to those of the genera Pleuracanthus and Orthacan- 

 thus of Agassiz ; so much so, that Beyrich suggests the propriety 

 of uniting the three genera as a subfamily of the Raiidce. The 

 late Prof. Goldfuss went a step further, and merged Xenacanthus 

 in Orthacanthus. It is therefore fair to presume, that the conti- 

 nental discovery has, given us the clue to the true characters of 

 the fish from which these peculiar spines are derived. In his lucid 



