MACROPOMA. 17 
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gill-rakers or calcified gill-supports. Of the opercular apparatus, only the 
operculum itself appears to occur. Its shape and proportions are shown in 
Pl. XXXV, fig. 9 (ep.), where it is only shghtly crushed. Its point of suspension 
is conspicuous, and its upper and anterior bevelled margins are a little sinuous. 
Its inner face is smooth, without any strengthening ridges (B. M. no. 115), and 
its outer face is always closely ornamented with smooth rounded tubercles. 
The pair of gular plates (Pl. XXXVII, fig. 9, gu.) completely covers the space 
between the mandibular rami, and the contour of one is best seen in fig. 10. 
Kach plate is ornamented, except in occasional smooth patches, with small 
rounded tubercles, which are very closely arranged, not in concentric lines. 
In the axial skeleton of the trunk there are no traces of vertebral centra, but 
the arches are well ossified, and they are especially stout in the fin-bearing part 
of the tail. Each of the neural arches and each of the hemals in the caudal 
region consist of a single piece, the two lamine of the arch being firmly fused 
with the spine, which is inclined to them at a shght angle. The total number of 
segments is uncertain, but is approximately as shown in the restored skeleton, 
Text-fig. 49, p. 173. In the abdominal region the neural arches are remarkably 
short, but in the caudal region they gradually lengthen until the longest occurs 
beneath the first ray of the caudal fin, whence they rapidly decrease in size 
backwards. The hemal arches in the caudal region are symmetrical with the 
opposed neurals, but those of the abdominal region must have been very small and 
short, none having hitherto been seen in the English Chalk fossils. Delicate short 
hemals are shown in the hinder half of the abdominal region of a specimen of 
M. speciosum from the Turonian of Bohemia (B. M. no. P. 9007). There are no 
calcified vertebral arches in the short extension of the trunk beyond the insertion 
of the caudal fin, which is seen in Pl. XXXVI, fig. 3 (¢.). 
Immediately below the anterior half of the vertebral column, extending 
throughout the length of the body-cayity, the ossified air-bladder is always 
conspicuous (Pl. XXXVI, figs. 1, 2, bl.). It consists of large, round, or cycloid, 
thin lamine, which are smooth on both faces, but sometimes exhibit a little 
crimping when crushed. These laminz are deeply overlapping from behind 
forwards, or in a direction opposite to that of fish-scales; and they are in such 
loose contact that they are often displaced in the fossils. They are in three paired 
longitudinal series, each consisting of eighteen to twenty components. As 
described by W. C. Williamson (loc. cit., 1849), they are composed of a pile of 
“horizontal lamellze, between which are developed large lacune, identical with 
those found in the bones of the endoskeleton (Text-fig. 50). These lacune not 
only distribute their large canaliculi in the plane of the lamelle, but shorter 
vertical twigs penetrate the lamelle, and thus keep up a communication between 
the inner and outer surfaces of the bladder. Some of the external lamelle lose 
their exact parallelism with those below, and one in particular assumes an 
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