482 Dr. R. 11. Traquair on Fossil Fishes 



British Fossil Vertebrata/ rejected Mesacanthus as a genus, 

 and restored its species to Acanthodes. 



I take this op[)ortunity of expressing my dissent from this 

 view, and to point out that, althougli the presence of inter- 

 mediate ventral spines is to ray mind quite sufficient for 

 generic distinction, it is not the only important difference 

 between those Old Red species and the true Acanthodes of the 

 Carboniferous and Permian formations. A glance at the 

 outlines of Acanthodes and Mesacanthus given in figures 1 and 

 2 will suffice to bring out the following remarkable distinc- 

 tions in the position of the fin-spines. In Acanthodes the 

 dorsal and anal spines arc situated proportionally nearer the 

 caudal fin than in Mesacantlnis^ while the ventral spines are 

 small and situated remotely from the anal, so that the ventral 

 fin itself forms a long low fringe ; while in Mesacanthus, on the 

 other hand, the ventral spines are nearly as large as the anal, 

 and situated considerably nearer to it tl.an to the pectorals. 

 The remarkable fact is therefore that in Mesacanthus it is the 

 small intermediate spines, and not those usually reckoned as 

 " ventral," Avhich correspond in size and position to the 

 ventral spines in Acanthodes, the idea being indeed almost 

 suggested that in tiie former genus the additional spines are 

 the posterior and not the anterior pair situated on the belly. 

 I do not propose to maintain such a theory, but certainly I 

 must hold that the larger size and different position of the 

 ventral spines, together with the presence of the intermediate 

 pair, are ample grounds for the generic separation of Mesa- 

 canthus from Acanthodes. 



3. Cheiracanthus Murchisoni, Ag. — Several specimens of 

 a Cheiracanthus have occurred which I refer to Ch. Murchi- 

 soni on account of the form and proportional size of the spiues, 

 though the scale-ornament is not preserved. 



4. Diplacanthus striatus, Ag. — Three specimens clearly 

 identifiable with this, the common Diplacanthus of the Orkney 

 as well as of the Moray- Firth beds. 



5. Rhadinacantlius longispinns (Ag.). — Several fragments 

 showing the characteristic spines and scale-ornament. 



Messrs. Woodward and Shcrborn have in their work 

 already quoted also rejected the genus lihadinacauthus which 

 I proposed for the Diplacanthus longispinns of Agassiz on 

 account of the apparent absence of the second or inner pair 

 of pectoral spiues, which are so conspicuous iu the typical 



