Bashford Dean 213 
Berlin specimen from which Fig. 12 was prepared. In any event they 
indicate that the ceratohyal region was more perfectly gill-bearing than 
in modern sharks.” 
That the mandibular arch in Acanthodes bronni shows segmentation 
must, accordingly, be admitted; and that its elements indicate homologies 
with the branchial arches. On the other hand, unfortunately, these 
homologies cannot be accepted as final, for the following reason: The 
condition of the Permian species, A. bronni is not confirmed by the con- 
dition in several of the Lower Devonian forms. I refer particularly to 
such a specimen, for example, as that of Ischnacanthus gracilis’ (Fig. 
13), in which no separate elements can be distinguished; and this is 
true, also, in a well-preserved jaw arch of Chetracanthus murchisont 
(Edinburgh). For it is obvious that if the earher Acanthodians show no 
trace of these elements, the condition in the much later forms may, like 
Cope’s “elements” in the crania of Xenacanths, be interpreted as 
artifact. 
As far as the writer is aware the roofing of the Acanthodian skull with 
dermal elements has never been described in detail. Its interest, how- 
ever, is patent, in as much as it represents an early, if not the earliest, 
form of strengthening the brain capsule in the gnathostome series. In 
certain genera it undoubtedly forms an effective shield for the brain, 
although, morphologically speaking, it must be regarded as but a paral- 
lelism of the dermal head shield of the higher fishes. For it consists not 
of a series of plates, each formed of fused shagreen elements, as one 
knows it in the ontogeny of recent fishes, but of a series of single, 
although greatly enlarged shagreen elements. Thus we see, for ex- 
ample, in Climatius scutiger® (Fig. 14), that the broad head roof is pro- 
tected with dermal plates, numerous (a score or more), arranged irregu- 
larly, not closely oposed to one another, and clearly not to be regarded 
as the homologues of parietals, frontals, pre- and postorbitals, ete. From 
their shape and radial ornamentation, they are obviously to be compared 
with the enlarged dermal denticles of many other elasmobranchs, e. g., 
rays.’ , It is evident, furthermore, from a comparison of the head-roofing 
* Dollo’s recent suggestion, ’06, that Acanthodians are plankton-eating forms, 
is recalled by these lamine—if they be interpreted as branchiostegal, and not 
as traces of gill filaments. 
*Powrie Collection, No. 258, Edinburgh. 
S Brit. Mus. 35,908. 
°Cf. in this regard (Fig. 15) an enlarged dermal denticle from the head- 
roof of Climatius reticulatus in the Edinburgh Museum. 
