190 
ACTINOPTERYGTI, 
the teeth apparently without vertical successors. Notochord per¬ 
sistent, destitute of ossifications in its sheath. Fin-rays robust, 
the majority well-spaced and articulated; fulcra absent, except 
perhaps quite at the base of the caudal fin ; dorsal and anal fins 
more or less extended. Scales rhombic when present, frequently 
wanting on the whole or part of the caudal region; almost in¬ 
variably strengthened by the inner rib on their anterior margin 
and united by a peg-and-socket articulation in connection there¬ 
with. 
The cranial osteology of the Pjnmodont fishes is as yet very 
imperfectly known. The little information available suggests that 
there was as much variation in this group as in certain aberrant 
tribes of modern fishes, such as the Plectognaths and Gymnodonts ; 
and in any general description it is therefore necessary to state 
precisely in which genus or genera several of the characters have 
been observed. When this statement is not made in the following 
account, the character in question has been met with very widely 
and may be regarded as normal in the family. The best-preserved 
specimens hitherto obtained are those of Mesturus leedsi from the 
Oxford Clay of Peterborough. The laterally-compressed examples 
of Gyrodus and Mesodon, from the Lithographic Stone of Bavaria and 
France, are also important; while the unique specimen of Ano- 
moeodus ivilletti from the Sussex Chalk (see p. 263) reveals for the 
first time the pterygo-palatine arcade. 
The chondrocranium is well ossified and always completely 
covered with membrane-bones, there being no vacuities in the 
cranial roof. The basicranial axis is more or less parallel with the 
roof behind the angulation of the frontal profile, and is then bent 
sharply downwards to the dentigerous oral face of the vomer ; while 
the latter is finally directed in a plane almost parallel with the 
first. The axis of the skull as a whole is directed downwards 
and forwards, making a considerable angle with the axis of the 
trunk. 
Of the chondrocranial elements, only the postfrontal (sphenotic), 
mesethmoidal, and supposed basisphenoidal bones have hitherto 
been clearly observed. The postfrontal in Mesturus is small and 
docs not appear on the cranial roof ; nor is there any external 
ornamented plate fused with its outer face. Its lower extremity- 
meets the basipterygoid process of the parasphenoid in a deeply 
interdigitating suture; while its posterior outer face takes part in 
the facette for the upper end of the hyomandibular. The mesetli- 
moid is always conspicuous as a deep thin lamina extending forwards 
from the anterior border of the orbital cavity and articulating below 
