126 
SELACHII, 
broad, flat, tessellated, in five or more series, the middle being the 
largest, the first, or first and second, lateral series on each side some- 
what smaller, and the others approximately as broad as long ; the 
dentition of both jaws strongly arched antcro-posteriorly. Dental 
crown smooth or slightly striated ; attached surface of tooth longi- 
tudinally ridged and grooved. Tail with dorsal fin in front of a 
barbed spine. 
The crowns of the teeth, when unworn, are usually very thick, 
but, after having been long in function, they become relatively thin. 
The thickness of the teeth, therefore, sometimes noted in specific 
descriptions, does not furnish a reliable diagnostic character. 
Rhinoptera daviesii^ sp. nov. 
Type. Portion of dentition (PI. III. figs. 6, 6 «) ; British Museum. 
Teeth transversely channelled, arranged in nine antero-posterior 
series. Each tooth of the median row is about four times as broad 
as long ; the teeth of the first and second lateral rows are also much 
transversely elongated, being respectively about three and two-and-a- 
half times as broad as long ; the teeth of the two outer rows are 
slightly broader than long. 
Form. ^ Loe. London Clay (Lower Eocene) : Isle of Sheppey. 
P. 1514. Type specimen, gencrically determined by Mr. William 
Davies. The fossil consists of a large portion of the denti- 
tion with parts of the ptorygo-quadrate and mandibular 
cartilages, embedded in hard clay. The teeth of one jaw 
are much scattered and displaced, but those of the other 
are scarcely disturbed, being shown in transverse section 
surrounding the cartilage, and more than half exposed from 
above. The upper aspect of the dentition, as far as pre- 
served, is shown of the natural size in PL HI- ® > 
the crowns of the teeth have the appearance of being chan- 
nelled in the direction of their long axes, and they exhibit 
a curious mode of interlocking antero-posteriorly, well seen 
in the transverse fracture (PI. III. fig. 6 a). The dental 
crown is thin and its lower portion is produced into a pro- 
jecting ridge anteriorly, which fits into a corresponding 
groove upon the posterior face of the tooth immediately iu 
front, and is firmly held by a small projecting ledge ap- 
parently from the root of that tooth. Such an arrange- 
ment has already been described by Agassiz in It. studeri . 
The root exhibits the usual antero-posterior grooves and 
ridges. Eyerton GoU. 
'■ Poiss. Foss. Tol. iii, p. 333, pi, R. flg. 4. 
