128 
SELACHII. 
flat, relatively very broad, in a single antero-posterior series. The 
upper dentition is strongly arched antero-posteriorly, with the teeth 
slightly and irregularly curved ; the lower dentition is flat, and the 
teeth are either straight, or more or less angularly bent in the middle. 
Dental crown smooth or slightly striated; attached surface of root 
longitudinally ridged and grooved. 
In this genus there is much less variability in the relations of the 
length and breadth of the teeth according to age than in Myliobatis; 
but the form of the lower teeth is so inconstant, that species cannot 
be determined upon the evidence of the lower dentition alone. 
As already recognized by Delfortrie 1 , Le lion’s genus Goniobcitis 
is founded upon a fragment of the lower dentition of the present 
form. 
Aetobatis irregularis^ Agassiz. 
1843. Aetobatis irregularis, L. Agassiz, Poiss. Foss. vol. iii. p. 327, 
pi. 47. figs. 3-5. 
1850. Aetobatis irregularis, F. Dixon, Foss. Suss. p. 200, pi. x. figs. 6-8, 
pi. xi. figs. 2-4. 
1850. Aetobatis rectus, F. Dixon, op. cit. p. 201, pi. xi. fig. 8. 
(P) 1885. Aetobatis irregularis, F. Noetling, Abh. Geol. Specialk. Preuss. 
u. Thiiring. Staaten, vol. vi. pt. 3, p. 27, pi. ii. figs. 4, 5. 
Type. Fragment of upper dentition ; British Museum. 
Dental plates of the upper jaw with a relatively low crown, ex¬ 
hibiting scarcely any elevation in the median portion ; the anterior 
and posterior border of each tooth usually more or less irregular, and 
the extremities gradually curved backwards. 
A detached tooth has been described by Dixon as A. rectus on 
account of its being somewhat straighter than the majority referable 
to this species ; but the differences are very slight and appear to be 
diminished by intermediate specimens, so that we venture to regard 
it as a variety. 
Form. Sf Loc. London Clay (Lower Eocene): Isle of Sheppey. 
Bracklesham Beds (Middle Eocene): Bracklesham Bay, Sussex. 
Barton Clay (Upper Eocene): Barton Cliffs, Hampshire. 
P. 3054. Type specimen, figured and described by Agassiz, loc. cit.; 
Isle of Sheppey. Enniskillen Coll. 
38836. Portion of upper dentition ; Isle of Sheppey. 
Boiuerbcink Coll. 
38835. Portions of upper and lower dentition, naturally associated ; 
Isle of Sheppey. Boiverbank Coll. 
1 Actes Soc. Linn. Bordeaux, yol. xxviii. 1871, p. 229. 
