80 
ACTIXOPTERYGII. 
The teeth are hollow and the calcigerous tubes in the dentine 
radiate from the central cavity. The cap of enamel is thick, and 
when the germ of the successional tooth is first formed it lies in 
exactly the opposite direction to that of the functional tooth, thus 
making a revolution of 180° while the root of the old tooth is 
absorbed and it prepares to appear. The marginal teeth, forming 
a single series on the maxilla, premaxilla, and dentary, are com¬ 
paratively small and prehensile ; those within are more robust and 
form a tritoral pavement. The latter are arranged with more or 
less regularity on the coalesced vomers, the pterygo-palatine arcade, 
and the splenial. In the earlier species all the teeth exhibit 
comparatively elongated pedicles, wdiile in the later and more 
specialized forms the inner teeth are fixed on very short bases. 
The opercular apparatus is complete and there is a good series of 
branchiostegal rays, the uppermost very broad ; but no indications 
of a gular plate have yet been discovered. The last-named fact is 
all the more remarkable, since in some of the other genera of 
Semionotidae the gular plate is relatively large and robust. The 
preoperculum is a long narrow bone, exposed throughout its 
length and much bent forwards below. The operculum exhibits 
a sharp elevation on its inner face for articulation with the 
hyomandibular process; its upper border is overlapped by the 
supratemporal plates, and its lower border deeply overlaps the 
suboperculum. The latter element is almost sickle-shaped, with a 
very large upwardly-directed process at its antero-superior angle, 
and abruptly truncated in front for union with the large elongate- 
triangular interoperculum. 
The branchial arches are delicate and have only been observed in 
a fragmentary state. The gill-filaments are shown in the Oxfordian 
species to have been supported by a series of long slender rods, 
apparently quadrate in section, and each serrated on one margin; 
the gill-rakers are small, stout, and pointed, tipped with enamel, 
more or less falciform, and arranged in well-spaced series. 
The endoskeleton of the trunk is well ossified, but owing to the 
thickness of the squamation it is rarely seen and is as yet only 
imperfectly known. The notochord is persistent, and in the earlier 
and smaller species no ossifications have been observed in its sheath ; 
but in the large "Wealden, and perhaps also in the large Oxfordian, 
species there are distinct indications of ring-vertebrae, at least in 
the abdominal region. The rings are much deeper than broad, but in 
the few known specimens ( e . g. nos. 2401, P. 1124, P. 6348 c ) their 
precise characters are obscure, and it can only be stated that each 
ring appears to consist of four sectors, the lower pair bearing short 
processes for the support of the ribs. The right and left halves of 
