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concave vertebre, to the Ichthyosaurus; while in the bony palate 
there is a deviation from the Batrachian character, and a resemblance 
to the Lacertian type. Another marked peculiarity in this fossil is 
the anchylosis of the base of the teeth to distinct and shallow sockets, 
by which it is made to resemble the Sphyrena and certain other 
fishes. From the absence of any trace of alveoli of reserve for the 
successional teeth, Mr. Owen believes the teeth were reproduced, as 
in many fishes, especially the higher Chondropterygii, which formed 
the Amphibia natantes of Linnzus, in the soft mucous membrane 
which covered the alveolar margin, and subsequently became fixed 
_ to the bone by anchylosis, as in the Pike and Lophius. No remains 
of the locomotive organs of the L. leptognathus have yet been found. 
Labyrinthodon pachygnathus.—In detailing the remains of this spe- 
cies, consisting of portions of the lower and upper jaws, an anterior 
frontal bone, a fractured humerus, an ilium with a great part of the 
acetabulum, the head of a femur, and two unguial phalanges, Mr. 
Owen dwells on further Batrachian characters and certain peculi- 
arities of structure, and shows the points in which it agrees with the 
L. leptognathus. A portion, nine and a half inches long, of aright . 
ramus of a lower jaw is first described; and in addition to the cha- 
racters common to it and the fragment of the lower jaw of the 
LL. leptognathus, in the structure of the angular and dentary pieces, 
the author shows that the outer wall of the alveolar process is not 
higher than the inner, as in Frogs and Toads, the Salamanders and 
Menopome, in all of which the base of the teeth is anchylosed to the 
inner side of an external alveolar plate. ‘The smaller serial teeth 
are about forty in number, and gradually diminish in size as they 
approach both ends, but chiefly so towards the anterior part of the 
jaw. The sockets are close together, and the alternate ones are 
empty. The great laniary teeth were apparently three in each sym- 
physis, and the length of the largest is considered to have been one 
and a half inch. A section through the base of the anterior tusk 
above the socket exhibits the structure described in Mr. Owen’s first 
memoir ; but a section of the second tusk, also taken above the socket, 
exhibited avery simplified modification of the labyrinthic arrangement, 
presenting a disposition closely analogous to that at the base of the 
teeth of the Ichthyosaurus. The apical half of the tusks has a smooth 
and polished surface, and the pulp-cavity is continued, of small size, 
into the centre of this part of the tooth. In the serial teeth, which 
in other respects, except size, correspond with the preceding descrip- 
tion of the tusks, the central pulp-cavity is more quickly obliterated, 
but the alveoli are large, moderately deep and complete: the texture 
of the teeth is dense and brittle. The base of each tooth is anchy- 
losed to the bottom of its socket, as in Scomberoid and Sauroid 
fishes; but the Labyrinthodon possesses, Mr. Owen says, a still 
more ichthyic character in the continuation, preserved in this speci- 
men, of a row of small teeth anterior and external to the two or three 
larger tusks. A double row of teeth thus occasioned does not exist 
in the maxillary bones, either superior or inferior, of any Batra- 
chian or Saurian reptile; in Mammalia it has been noticed only in 
