194 
ACTINOPTERYGII. 
symphysis, and bearing a row of prehensile teeth. An angular 
plate, though not extending upwards quite to the oral border, 
completes the posterior part of the outer face of the mandibular 
ramus ; and its hinder ascending portion meets a small coronoid 
bone, with which it frequently becomes fused. The hyoid elements 
are not satisfactorily known, though in Mesturus verrucosus a short 
and deep ceratohyal with a small hypohyal is evidently observed 
(PI. XV. fig. 1, ch., %.). 
The opercular and branchiostegal apparatus is remarkably 
reduced. The operculum is relatively small, deep, and narrow, 
truncated above where it meets the postero-lateral edge of the 
cranial roof, and tapering below to a point. This bone has hitherto 
been named “ supraclavicle,” but its true nature is indicated both 
by a specimen of Gyrodus heoccigonus (PI. XYI. fig. 3) and by one 
of Mesturus leedsi (no. P. 6834). The preoperculum (commonly 
termed “ operculum ”) is very large, triangular in shape and much 
expanded below; its mode of attachment to the mandibular sus- 
pensorium is indicated in the specimens both of Anomoeodus ivilletti 
and Gyrodus hexayonus already referred to. The suboperculum 
and interoperculum are wanting; and only two branchiostegal 
rays have been observed immediately below the preoperculum in 
Mesturus leedsi and in Gyrodus frontatus (PI. XYI. fig. 2). In 
Gyrodus and Mesturus the space between the rami of the mandible 
is completely covered with small polygonal tesserae, there being no 
gular plate. On the branchial arches, long and slender calcified 
gill-filaments, more or less denticulated, are often observed. 
Teeth are confined to the vomer, splenial, premaxilla, and dentary. 
They are all hollow, with a short base firmly anchylosed to the 
supporting bone; and the present writer has never observed any 
provision for replacement. The only suspicion of such an arrange¬ 
ment is afforded by the Oxfordian specimens of Mesturus leedsi. 
The teeth of the premaxilla and dentary are prehensile and arranged 
in a single series ; those of the vomer and splenial are tritoral and 
form an extended pavement. Anteriorly the teeth of these pave¬ 
ments are nearly equal in size and do not exhibit any very definite 
arrangement; but posteriorly as a rule they soon become differ¬ 
entiated and disposed in regular longitudinal series of more or less 
different sizes. Appearances suggest that the splenial and vomerine 
elements are continually being lengthened by growth behind, the 
only new teeth obtained during the life of the animal being those 
added in this manner. If so, it is much the same kind of growth 
as that which takes place in the dentition of the Cochliodontidae and 
Myliobatidae among Elasmobranchs : the only differences being due 
