from Achanarras Quarry , Caithness. 
481 
Remarks on the foregoing Species . 
1. Dipterus Valenciennesii . — This is by far the most com- 
mon fish at Achanarras, and, so far as the external characters 
of the species are concerned, the specimens are the most 
beautiful which I have seen from any locality in Scotland. 
They occur from 6 to 15 inches in length. 
2. Mesacanthus , sp. — Specimens of a small Mesacanthus 
are not uncommon, but hardly in a sufficiently good state of 
preservation for accurate identification as to species. In size 
and the commonly twisted or contorted condition of the body 
they resemble M. pusillus y Ag., of the Moray-Firth beds. 
The genus Mesacanthus was instituted by myself in 1888* 
for those small species previously referred to Acanthodes (A. 
pusillus , A g v A. Mitchelli , Eg., A. Peachii , Eg.) which, 
Fig. 1. 
Outline of Acanthodes sulcata s, Ag., much reduced. 
p , pectoral spines ; v, ventral spines ; a , anal ; d, dorsal 
Fig. 2. 
Outline of Mesacanthus Mitchelli , Eg., natural size. 
i. sp., intermediate spines ; the other letters as in fig. 1. 
though resembling that genus in having one dorsal spine 
placed posteriorly to the anal, have in addition a pair of 
minute intermediate spines on the belly between the pectoral 
and ventral ones. Messrs. Smith Woodward and Sherborn 
have, however, in their recently-published c Catalogue of 
* Geol. Mag. dec. iii. vol, v. p. 511, 
