246 
SELACHII. 
not exceed its maximum breadth. The fossil, in fact, is very similar 
to the dermal tubercles of Hybodus (PI. Till. figs. 2,3), and though 
regarded as a tooth by M‘Coy, L. G. de Koninck, and J. W. Davis, 
it may be reasonably assigned to the outer skin, as already suggested 
by Newberry & Worthen, Trautschold, and Lohest. In accordance 
with this determination, Trautschold proposes to substitute the name 
of Ostinasjhs for that of Petrodus, as being more appropriate h 
The following examples of Petrodus are comprised in the Col¬ 
lection :— 
P. 2244. 
Two tubercles from the Carboniferous Limestone of Derby¬ 
shire, similar to those in the Woodwardian Museum, 
Cambridge, named P. pcitelliformis , M‘Coy 2 . The ex- 
serted portion has a pointed apex, and the radiating 
ridges are about 13 or 14 in number, terminating at the 
base, very strong, sometimes dichotomously branching, 
and sometimes slightly marked by transverse sulci. 
Egerton Coll. 
46823-4. Six similar specimens, and piece of limestone exhibiting 
six others ; Yoredale Docks, near Todmorden, Lancashire. 
Gilbertson Coll. 
P. 2863. Three almost similar tubercles, of the form named P. occi-_ 
dentalis , Newberry & Worthen 3 ; Coal-Measures, Belle-* 
ville, Illinois. EnnisJcillen Coll.' 
P. 4734. Small example, with comparatively smooth ridges, referable 
to P. barbotanus, H. Romanowsky 4 ; Carboniferous Lime¬ 
stone, Mjatschkowa, near Moscow, Russia. 
Purchased , 1884. 
P. 5855. Smoother tubercle; Yoredale Rocks, near Todmorden. 
Presented by S. Barker, Esq., 1888. 
P.5116. Live larger specimens ; Mjatschkowa. Purchased, 1886. 
P. 4734 a. Iwo tubercles, oval in shape, not pointed, but with 
1 Nouv. Mem. Soc. Imp. Nat. Moscou, vol. xiii. (1874), p. 29S. 
2 Ann. Mag. Nat. Hist. [2] vol. ii. (1848), p. 132, and Brit. Pakeoz. Foss. (1855), 
p. 637, pi. 3 g. figs. 6-8 ; J. W. Davis, Trans. Roy. Dublin Soc. [2] vol. i. (1883), 
p. 400, pi. li. fig. 16; M. Lohest, Ann. Soc. G-eol. Belg. vol. xi. (1883), p. 318,’ 
pi. iii. figs. 4-6, pi. v. fig. 1. 
d Pal. Illinois, vol. ii. (1866), p. 70, pi. iv. figs. 15, 16. 
4 Bull. Soc. Imp. Nat. Moscau, 1864, pt. ii. p. 164, pi. iv. fig. 36; Tubercule isoU 
dune plaque de V Asterolepis (?), H. Romanowsky, loc. cit. p. 170; Ostinaspis bar - 
botana, H. Trautschold, Nouv. Mem. Soc. Imp. Nat Moscou, vol. xiii. (1874), 
p. 298, pi. xxviii. fig. 12, a-d. 
