ox THE ANATOMY OF FISHES. 
177 
US well, and except for a partial intervertebral suture between the fourth and fifth, 
are indistinguishably fused together. Each of the transverse processes of the fourth 
vertebra is said by Eamsay Wright to consist of a dorsal and a ventraP' lamina, 
the former having its origin from the neural arch and the latter from the centrum 
of the complex vertebra. The two plates approximate and face posteriorly, but 
remain widely separated anteriorly so as to enclose a cavity between them which is 
open distally in the dry skeleton, but closed in front by the bones forming the 
posterior face of the skull, and encloses the rudimentary air-bladder. Not only are 
the skull and vertebral column firmly anchylosed together, but the three anterior 
vertebral centra, that is to say, the first and the anterior portion of the complex, 
and those parts of the Weberian mechanism usually associated with them, including 
the claustra, scaphia, intercalaria, the anterior portions of the tripodes, the saccus 
paravertebralis and the atrial cavities, are telescoped, as it were, on to the upper 
surface of either the basioccipital or exoccipitals, according as they are median or 
paired lateral structures, and consequently lie altogether within the hinder jiart of 
the cranial cavity. The centrum of the first vertebra is situated on the cranial 
surface of the basioccipital, within the foramen magnum, and on the dorsal surface of 
the former are placed the two atrial cavities, excavated in the substance of a thick 
mass of dura mater, and separated from each other by a median partition of the same 
material. The roof of the cavum sinus imparls is only partially ossified as a thin 
plate of bone entirely distinct from the exoccipitals. The floor of the cavum is 
formed by the basioccipital and exoccipitals, but in consequence of the forward dislo- 
cation of all the structures concerned the scaphia form its lateral walls, instead of 
those of the atrial cavities, as is the case with nearly all other Siluroids. 
A complete series of Weberian ossicles is present, including the claustra, but no 
special description of them is given. 
• The air-bladder is completely’" divided into two laterally situated air-sacs, each of 
which lies in the cavity enclosed by the so-called dorsal and ventral lamina3 of the 
corresponding transverse process of the fourth vertebra. Each sac is slightly con- 
stricted in the middle, and while the outer half is membranous, the inner is ossified 
in such a way as to form a bony cup, which is continuous at its outwardly directed 
free margins with the membranous portion, but at its closed base is confluent with 
the arch and centrum of the complex vertebra. The osseous half of the bladder is 
evidently the result of the ossification of the tunica e.xterna, and its subsequent 
fusion with the wall of the neural canal. Internally the entire bladder is lined with 
tunica interna. A connection between the hinder end of the tri[)us and the meni- 
* \Vc have elsewhere given reasons for the belief that the so-called ventral lamina is really no part of 
the transverse process, but ought rather to be reguKled as the ctjuivalcnt of the ventral process of 
llnyarius, and, like the latter, probably owes its formation to the extension of ossihed deposit from the 
superficial ossifications of the complex centrum into the venti'al portion of the superficial coat of the 
air-bladder. (See Morphological Summary, p. 224). 
MDCCCXCIII. — 13 . 2 A 
