482 Dr. R. H. Traquair on Fossil Fishes 
British Fossil Vertebrata,’ rejected Mesacanthus as a genus, 
and restored its species to Acanthodes. 
I take this opportunity of expressing my dissent from this 
view, and to point out that, although the presence of inter- 
mediate ventral spines is to my 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 are situated proportionally nearer the 
caudal fin than in Mesacanthus , 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 than to the pectorals. 
The remarkable fact is therefore that in Mesacanthus it is the 
small intermediate spines, and not those usually reckoned as 
a ventral,” which correspond in size and position to the 
ventral spines in Acanthodes , the idea being indeed almost 
suggested that in the 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. Gheir acanthus 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 spines, 
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. Rhadinacanthus longispinus (Ag.). — Several fragments 
showing the characteristic spines and scale-ornament. 
Messrs. Woodward and Sherborn have in their work 
already quoted also rejected the genus Rhadinacanthus which 
I proposed for the Diplacanthus longispinus of Agassiz on 
account of the apparent absence of the second or inner pair 
of pectoral spines, which are so conspicuous in the typical 
