XXII 
INTRODUCTION. 
attain to a considerable size (e. g., Or acanthus) ; and these may have 
been arranged upon the ventral surface (like the spines of Acan- 
thodians), or may perhaps have occupied the postero-lateral angles 
of the head (like the cornua of Cephalaspidians); some, however, 
are long and slender, and seem to have armed the front margin of 
the pectoral or pelvic fins. In the Mesozoic Hybodus , Acroclus, and 
Aster acanthus, two pairs of large hooked spines upon broad bases 
also occupied the lateral regions of the head ; but no discovery of 
paired fin-spines in deposits later than the Permian has yet been 
recorded. 
Dentition . 
Pointed teeth and obtuse teeth occur among the earliest Elas- 
mobranch fossils ; but the former, as well as the latter, are firmly 
articulated together, and must always have formed part of a 
dentition in which several series were functional. Though the 
teeth of Cladoclus and Dvploclus are as sharply pointed as those of 
most recent Sharks, the piercing crown is placed upon a broad 
horizontallj’-expanded base, permitting of a considerable amount of 
interlocking between one tooth and another—an arrangement most 
nearly parallelled in the surviving Chlamycloselache. It is evident, 
indeed, that all the modern types of dentition, in which not more 
than one or two series of teeth are simultaneously functional, are 
highly specialized modifications of this primitive arrangement; and 
the change residts from the deepening and lateral compression of 
the root of each tooth, rendering its base of support less fixed, and 
often not permitting its coming into use until after attaining the 
summit, or passing to the outer side, of the jaw-cartilage. 
In rare instances, the stages of this interesting course of specializa¬ 
tion can already be traced to a certain extent; and no case is more 
striking than that of the genus Notidcinus h In the earliest known 
Jurassic species, the teeth possess few coronal cusps, fixed upon a 
stout, depressed, and backwardly-expanded base of attachment ; in 
the Upper Cretaceous species the crown is longer, and the root or 
base exhibits considerable lateral compression : and in the Pliocene 
species the tooth possesses the greatest number of cusps, and its 
root is both very deep and extremely compressed. If Orthacodus is 
rightly placed in the Lamnidse, this, too, is an illustration of the same 
principle. The Jurassic tooth just mentioned has a broad hori- 
1 Geol. Mag. [3] vol. iii. (1886). p. 257. 
