322 J. W. DAVIS ON THE GENUS PLEURACANTHTJS, AGASS. 



side rows of sharp, short, hook-shaped, backward-pointing teeth.'' 

 This is evidently the same as Agassiz's genus Pleuracanthus : but 

 Beyrich gives it the new generic name Xenacanthus, remarking that 

 the OrtJiacanthus Declienii of Goldfass must be given up in favour of 

 Xenacanthus, and that though the name Pleuracanthus of Agassiz has 

 priority, and would have been well suited to embrace this new fish, 

 it was too well known as the name of a spine only. 



In 1855, Sir Philip de M. Grey-Egerton, at the Glasgow meeting 

 of the British Association, pointed out that the spines of Pleura- 

 canthus and Xenacanthus and the Diplodus-teeth all belonged to the 

 same genus of fossil fish; and two years later, in the 'Annals and 

 Magazine of Natural History/ the same ichthyologist, considering 

 publication as the test of priority, enforces the claim of the genus 

 Pleuracanthus. 



Prof. Rudolph Kner*, in 1867, in an elaborate paper on the 

 genera Orthacanthus, Goldf., and Xenacanthus, Beyr., after a care- 

 ful examination of all the Bohemian specimens available, arrived at 

 the same conclusion as to their identity that had been put for- 

 ward twenty years previously by Beyrich. Notwithstanding this, 

 the specimens which have been figured in illustration of his views 

 embrace examples with spines of both the Pleuracanthus and Ortha- 

 canthus type. 



The principal difference between the genera Orthacanthus and 

 Pleuracanthus in the type specimens figured by M. Agassiz lies in 

 the position of the two rows of denticles or barbs. Both spines are 

 figured as straight, and have an internal cavity open at the base 

 and extending far towards the point. In Pleuracanthus the denticles 

 are situated on the lateral faces of the spine, the two rows being as 

 widely separated as possible ; whilst in Orthacanthus they are very 

 close together and extend along the posterior face. A reference to 

 the series of spines described in the following pages, along with 

 those already described and figured in the memoirs of the State 

 Survey of America, will prove that the difi'erence in the relative 

 position of the rows of denticles must either be of small generic 

 importance or that many new genera will have to be formed for 

 their accommodation. Almost every intermediate stage between the 

 two forms is now known; the denticles extend at every angle 

 between the sides and back of the spine. After careful consideration 

 of the specimens, one is led to the more natural conclusion that 

 they have been borne by fishes having characters of a single generic 

 type, and that they should consequently be included in the genus 

 Pleuracanthus, Agass. 



The teeth oi JDiplodus have hitherto been found associated in- 

 discriminately with the spines of Pleuracanthus and Orthacanthus; 

 and there has been no generic difference detected in the somewhat 

 numerous species of DipZoc^ws-teeth : this renders the probability of 

 the spines belonging to different species of the same genus very great, 

 and stamps their relationship almost with certainty. 



A short time ago I had the pleasure of describing two new species 

 * Sitzimgsberichte der kaiserl. Akad. der Wissensch. Band Iv. 1867. 



