488 Dr. A. Smith Woodward — A Carhoniferoiis Ichthyodorulite. 



large, smooth, radiating ridges, which terminate close to the basal 

 edge, leaving a very small margin for insertion in the skin. The 

 basal diameter of the largest tubercles is about equal to the basal 

 extent of the associated spines ; and there are no gradations 

 between the two series of structures. 



In part, at least, of the group just described Listracanthus is 

 nearly as abundant as Petrodus ; but in the second specimen 

 discovered by Mr. Ward there is only one imperfect spine among 

 the numerous tubercles. These tubercles, in comparatively soft 

 shale, are especially well shown, and two of the best examples are 

 represented in Figs. 6-8. It will be observed that their highest 

 point is not central but displaced towards one end of the ovoid. 



From the new specimens it is evident that Listracanthus is 

 a strangely modified dermal tubercle, occurring in considerable 

 numbers on part at least of the head or trunk of the fish to which 

 it belonged. There can also be no longer any doubt that some 

 varieties of the so-called Petrodus form part of the same armature, 

 as already suspected from discoveries in the Coal-measures of 

 Indiana, U.S.A.^ Although, indeed, there are no gradations 

 between the associated spines and tubercles in the specimens now 

 described, one of the Eussian Lower Carboniferous varieties of 

 Petrodus (Fig. 9) is essentially a squat Listracanthus with deepened 

 lateral ridges and an exaggerated basal expansion. The so-called 

 Petrodus acutus, from the Coal-measures of Illinois,^ is obviously 

 similar. The two forms of dermal appendage under considei'ation 

 are thus merely extreme modifications of one and the same type. 

 It is, however, quite possible that some of the Carboniferous 

 tubercles named Petrodus belong to fishes of quite another genus ; 

 for the typical Petrodus patelliformis is remarkably similar to the 

 dermal tubercles of Hybodus, and may well have belonged to an 

 ordinary Hybodont shark. 



This uncertainty as to the relationships of the various tubercles 

 known as Petrodus (M'Coy, 1848) renders it advisable to continue 

 the use of the comparatively modern generic name Listracanthus 

 (Newberry & Worthen, 1870) for the remarkable fossil here 

 discussed. Judged both by tlae peculiar spines, and also by the 

 tubercles, Mr. Ward's specimen represents a hitherto unknown 

 species, and it may be termed Listracanthus Wardi in honour of 

 its discoverer. 



It is interesting to note that, like the spines of Listracanthus 

 described by Mr. Herbert Bolton from the Lancashire Coalfield, 

 this new species was obtained from a truly marine band of the 

 Coal-measures. It was found in the shale underlying the Twist 

 Coal, Nettle Bank Colliery, Smallthorne. 



1 J. S. NeAvberry: Eep. Geol. Surv. Ohio, vol. ii, pt. 2 (1875), p. 56. 



* Newberry & "Worthen : Palseont. Illiuois, vol. ii (1866), p. 72, pi. iv, fig. 17. 



