178 FOSSIL FISHES OF THE ENGLISH CHALK. 
undulatory arrangement, in the folds of which, alternately above and below, are 
placed large irregularly-shaped lacune, the distorted prolongations from which 
are obviously modified canalicul. This curious structure is covered over with 
other more dense and apparently structureless lamella, which fill up the irregu- 
larities and restore the parallelism of the surface with the lower lamella, consti- 
tuting the exterior of the bladder. . . . No canals of any kind pass through 
the tissue.’ When well preserved, the air-bladder is seen to be largest in front, 
where it opens by a small median orifice (Pl. XXXVI, fig. 2, 0.); and it gradually 
tapers backwards to a bluntly-pointed, closed hinder end. 
In the pectoral arch the clavicle is much the largest element, and is not 
connected with any superficial, ornamented plate. It is long and narrow, gently 
arched; laterally compressed in its upper half, where it ends in a little rounded 
Fia. 50. Macropoma mantelli, Agassiz; microscopic structure of wall of air-bladder, magnified 350 diameters. 
A. Horizontal section of surface. 3B. Vertical section of complete thickness of wall. After W. C. Williamson. 
expansion ; and widened in its lower half, where it is convex in front, deeply 
hollowed behind (Pl. XXXYV, fic. 9; Pl. XXXVI fig. 9; Pl. XXXVIDL fig. 1, el.), 
and ends in a point. Above and within the upper end of the clavicle is another 
large flattened plate, of which the complete form is shown in the type specimen 
(Pl XXXVI, fig. 1, ptt.). It is also smooth, narrowest above and widest below, 
and specially remarkable for the slender, pointed process which is directed 
forwards from its middle. It may be interpreted either as supraclavicle or 
post-temporal, but agrees best in shape with the last-named bone in Polypterus. 
Below and in front of the lower end of the clavicle there is a small infraclavicle 
CET Oey i ne. 9 Pl XXXW I ie. 1, tis), whieh isa long and slender 
bone with a smooth triangular expansion at its incurved lower end where it 
meets its fellow of the opposite side. Behind the middle of the clavicle and 
overlapped by it in one specimen (Pl. XXXVIII, fig. 2) there occurs a thin lamina 
of bone which may probably be named postclavicular (pel.). It is longer than 
deep, deepest at each end, and slightly inclined upwards behind. Somewhat 
lower down, behind and within the expanded portion of the clavicle of other 
