242 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
posterior pillars which curve forv*^ards into the dorsal wall of the anterior chamber, 
and it is possible that the ossification of those fibres along the line of their skeletal 
attachment may have somewhat widened the lamina in some Siluroids {e.g., 
Macrones), but in many others {e.g., Platystoma, Malapterurus, &c.), where the 
dorsal lamina is represented in the usual position by a very slender spicule of bone, 
and the lateral portions of the posterior pillars are inserted more posteriorly than in 
Macrones, that is, to the ventral surfaces of the transverse processes of the fifth 
vertebra, it is probable that the lamina is formed solely by the ossification of the 
superficial coat of the air-bladder. 
There can be but little doubt that the radial nodules are independent ossifications 
of the tunica externa in relation with the skeletal attachments of the radially 
disposed fibres converging from the concavities of the crescentic processes of the 
tripodes towards the complex centrum, which may either permanently retain their 
close fibrous connection with the centrum or become anchylosed thereto. Ramsay 
Weight (43) has shown that the crescentic process of the tripus, or at all events the 
inwardly curved portion which is imbedded in the dorsal wall of the bladder, is also 
formed as an ossification of the tunica externa, and only secondarily becomes con- 
tinuous with the remainder of the ossicle. The same writer {loc. cit.) refers to the 
superficial ossifications investing the sides of the complex and fifth vertebral centra 
as “developed in connection with the air-hladder.” If this be so the ossifications 
are obviously due to the conversion into bone of the outer stratum of that part of the 
tunica externa of the dorsal wall of the anterior chamber which is moulded and 
adherent to the lateral and ventral surfaces of these centra, and the fact that the 
ossifications are always co-extensive with that part of the dorsal wall wdiich is 
represented by little more than the tunica interna, and certainly appear to replace 
the tunica externa in that region, confirms this conclusion.* It would seem, there- 
fore, a legitimate inference that these ossifications are due to the conversion into bone 
of the primitively more extensive, purely fibrous attachments of the posterior pillars 
to the lateral and ventral surfaces of the complex and fifth centrat in much the same 
way that the oblique lateral ridges and the dorsal laminre are ossifications in the 
skeletally attached portions of the superficial coat. It may be also concluded that all 
* Recently Sorensen (.37) has more precisely shown that in Platystoma the superficial ossifications 
do result from the ossification of the tunica externa in the way we have suggested, and in this Siluroid 
may even form the greater part of the complex centrum. The same writer is of opinion that the inner 
layer of the peritoneum (la plevre), or what we have referred to as the superficial coat of the bladder, is 
also concerned in the formation of these ossifications. As illustrating the contagious nature of this 
tendency to ossification, it may be mentioned that Sorensen further concludes that the lateral ridges 
bounding the aortic groove are, in part at least, due to the ossification of the walls of the aorta itself, 
which may, as in Platystoma, even lead to the formation of a complete bony aortic canal. 
f Sorensen’s statement (loc. cit.) that in Platystoma the fibres in the superficial ossifications are so 
disposed that they exactly coincide with the direction of the fibres forming the forward extension of the 
posterior pillars into tlie dorsal wall of the anterior chamber supports this suggestion. 
