84 Dr. R. H. Traquair—Carboniferous Selachir. 
Cestracion, while there is no trace of the segmented prolongation of 
the metapterygium which we saw in the HE. Kilbride Cladodus. 
This interesting specimen is fatal to Mr. I’. Stock’s idea that the 
spines in this genus were paired,’ as well as to its location among 
the Chimeroids as maintained by Prof. Hasse.? That it is a Hybo- 
dont cannot, in my opinion, be doubted. 
Orodontide, De Koninck. 
If the Mesozoic genus Acrodus be a Hybodont,—and its spines are 
generically indistinguishable from those of Hybodus,—it is difficult to 
draw any line between the Hybodontidee and Orodontidee. 
One of the genera which De Koninck and Mr. Davis place in this 
family must, however go, namely, Lophodus of Romanowski. 
Romanowski separated from Agassiz’s Helodus such forms as didymus, 
levissimus, mammillaris, as having one or more prominent elevations 
on the crown, and a well-developed compressed and_ vertically 
striated root, while he considered H. planus, which has no such root 
and no special elevation on its crown, to represent the old genus. 
Unfortunately both ‘“ Helodus” planus and ‘“ Lophodus” didymus 
belong to the mouth of the same fish, and that fish is Psephodus 
magnus! Moreover, as I have once remarked, if the old genus 
Helodus was to be divided, surely the characters of the type-species, 
H. simplex of the Coal-measures, ought first to be ascertained and 
duly considered. Now a fine series of specimens of Helodus simplex, 
Ag., in the collection of Mr. John Ward, F.G.S., Longton, clearly 
shows that the teeth in this species have the form of “ Lophodus,” 
that the entire dentition consisted of teeth generally similar in shape, 
and that the dorsal fins were armed with spines resembling those of 
Pleurodus. 
Whatever be the nature of the teeth which Mr. J. W. Davis retains 
in, and adds to Helodus, there can be no doubt that H. simplex must 
remain the type of Agassiz’s genus, in which also Chomatodus cinctus, 
Ag., ought to be placed, as already indicated both by McCoy and 
Davis. 
Cochliodontide. 
The closeness of the alliance between the Cochliodontide and 
Orodontidz is shown by the fact that the anterior teeth of Psephodus 
and Cochliodus are generically indistinguishable from those of Helodus. 
As seen in Psephodus, which is one of the least specialized of the 
Cochliodontidz, the posterior teeth lose their deep roots, become 
flattened, and tend to fuse together into broad inrolled plates. I 
have a specimen of the broad tooth plate of Psephodus magnus, Ag., 
which by a groove is divided longitudinally into two portions, which 
pretty closely represent not uncommon forms of Helodus planus. 
The grooves on Peecilodus, Deltodus, etc., also to my mind represent 
the morphological origin of those plates from the fusion of smaller 
1 Ann. and Mag. Nat. Hist. (5), xii. 1883, p. 188. 
* Natiirliches System der Elasmobranchier. 
