284 
SEtACHir. 
longitudinal keel, and with only faint indications of lateral denticles 
in one jaw; a median symphysial series present. 
As shown hy the series of specimens described below, there is 
considerable variation in the relative proportions of the successive 
rows of teeth, exactly as is to be observed in the dentition of the 
living Cestracioii. it seems probable that the two principal series 
of teeth were always relatively smaller in one jaw than in the 
opposite ; and this jaw is here provisionally regarded as the uj>per. 
Though the originals of Agassiz's pi. xxi. tigs. 4-7 are stated to be 
in the Enniskillen and Egertou Collections, they cannot now be 
identified. 
Fonn. ^ Loo. Lower Lias : Dorsetshire, Wiirtemberg, Somerset- 
shire, and (?) N. France. 
Unless otherwise stated, all the specimens were obtained from the 
neighbourhood of Lyme llegis, Dorsetshire. 
P. 3154. The crushed anterior portion of a fish of moderate size, 
exhibiting no definite outline, hut merely fragments ot 
cartilage associated with scattered teeth, shagreen, and 
the two dorsal fin-spines. The teeth exhibit the charac- 
teristic roiinduess and finely-marked coronal surface of 
those of A. noliili% and those from the anterior portion of 
. the jaw show traces of lateral prominences ; the largest 
measure 0-022 in length. The shagreen- granules pre- 
served are minute, slender and conical, longitudinally 
ribbed and grooved. The first dorsal s])inc is incomplete 
in the distal half and much broken, but the second dorsal 
is nearly perfect. The latter is comparatively short aud 
stout, measurit)g 0-255 in total length ; the lateral ribs 
are broad, lounded, and irregularly wavy ; and the x)os- 
terior face has the appearance of being slightly convex. 
Enniskillen Coll. 
P. 2736. A large portion of the dentition, figured, of the natural 
size, in PI. XI 11. fig. II. The specimen shows about forty- 
four teeth arranged in natural order, and comprises the 
symphysial region, portions of the first three rows on one 
side, and portions of live on the other. Throe small teeth 
(o) represent the median symphysial series, each mea- 
suring about 0-015 in length and exhibiting three lateral 
points. Ot the first lateral series (i.), the teeth measure 
0-018 in length, and arc unsym metrical, each having the 
greatest elevation near the anterior extremity, with three 
or four lateral j)oints, -widely separated behind, closely 
