512 Dr. R. H. Traquair—Old Red Sandstone Fishes. 
included Mesacanthus pusillus, Ag. sp., M. Peachii, Egert. sp. (incl. 
A. coriaceus, Egert.), and M. Mitchelli, Kgert. sp. 
Cheiracanthus, Agassiz.—In this genus a multitude of supposed 
species have been erected upon differences which I have for years 
been convinced are due merely to various conditions of preservation 
in different kinds of rock. As absolutely synonymous with the 
type species Ch. Murchisoni, Ag., I include Ch. microlepidotus, Ag., 
Ch. minor, Ag., Ch. lateralis, McCoy, and Ch. pulverulentus, McCoy. 
But Ch. grandispinus, McCoy, and Ch. latus, Egerton, are “good 
species,” and are easily recognizable wherever they occur. 
Diplacanthus, Agassiz.—The type of the genus, D. striatus, Ag., 1s 
weil known to possess two dorsal spines, four pectoral, two inter- 
mediate, two ventral, and one anal. Specifically, J am unable to see 
any valid distinction between D. striatus from Cromarty and Gamrie, 
D. striatulus, Ag., from Lethen and Tynet, D. crassispinus, Ag., and 
D. gibbus, McCoy, from Orkney; all the distinctions which have 
been noted are to my mind due to differences of age and mode of 
preservation. 
Rhadinacanthus, n. gen. Traq. (= Diplacanthus, Ag. pars.).—- 
The long, slender dorsal spines of Diplacanthus longispinus, Ag., 
contrast most strongly in appearance with those of D. striatus, but 
a more obvious mark of generic distinction is seen in the absence 
of the second or lower pair of pectoral spines. There is a pair of 
intermediate ventral spines between the pectorals and ventrals. As 
a synonym of Rhadinacanthus longispinus, Ag. sp., 1 feel constrained 
to include Diplacanthus gibbus of McCoy. 
Ischnacanthus, Powrie.—This genus, proposed by Powrie’ for 
Diplacanthus gracilis, Eger., and afterwards withdrawn by its founder,” 
ought to be restored, as this beautiful Forfarshire species is destitute 
of the intermediate ventral spines with which all the other Diplacan- 
thoid fishes are provided ; the powerful dental armature of its jaws 
is also a well-marked feature of the genus. 
Suborder CrossoprEeRYGII. 
Family Honoprycuups”. 
The Holoptychiide emphatically differ from the other Crosso- 
pterygian families in having the pectoral fins acutely lobate; the 
internal skeleton of this fin must have been entirely cartilaginous, 
but there can be no doubt that it was a “ biserial” archipterygium 
as in Ceratodus; the ventrals, however, are only subacutely lobate. 
The teeth are dendrodont in structure, although, as in the Rhizodontide, 
the Janiaries of the mandible, except the most anterior, are set on 
separate internal dentary ossicles. In the Rhizodontide on the other 
hand the pectorals are subacutely or obtusely lobate, the internal 
skeleton of both pectoral and pelvic limbs forming an abbreviate 
uniserial archipterygium, and the teeth are “labyrinthodont” in 
structure, the dentine of the base being vertically folded in a 
more or less complex manner, so as to divide the pulp cavity into 
1 Quart. Journ. Geol. Soc. 1864, p. 419. 
2 ‘Trans. Edinb. Geol. Soc. vol. i. p. 289. 
