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PROFESSORS T. W. BRIDGE AND A. C. HADDON 
contiguous ventral margins of the basioccipital and the centra of the first and complex 
vertehrse. In some Siluroids the same result is secured bj the downward growth and 
coalescence of the accessory processes, to form a stout subvertebral process. In other 
Siluroids the connection of the skull with the anterior vertebrae may be rendered still 
more intimate by the articulation of the spine of the supraoccipital with the expanded 
dermal plates of the first and second interspinous bones {e.g., Auchenipterus, Oxydoras, 
&c.), or even by the extension of bony lamime between the supraoccipital and the 
dorsal surfaces of the transverse processes of the fourth vertebra {e.g., Arius, Ba- 
trachocephcdus, Osteogeniosus, and Ketengus). 
The centrum of the first vertebra varies greatly in size, but is always smaller than 
any of the other anterior vertebral centra. Two pit-like sockets are always found on 
its dorsal surface for the reception of the condylar processes of the scaphia. The 
complex and fifth centra are the largest, or at all events the longest, of the anterior 
centra, and, as a rule, their anterior and posterior concavities are unsymmetrically 
developed. In nearly all cases these centra are not only elongated, but laterally 
compressed, so as to form a prominent subvertebral keel, which gives rise to a deep 
groove along the medio-dorsal line of the anterior chamber of the air-bladder, and, at 
the same time, internally, to a well marked longitudinal ridge partially subdividing 
the cavity of tlie chamber into two laterally bulging halves. As a rule the centrum 
of the complex vertebra is much longer than the centrum of the fifth vertebra, although, 
exceptionally, it may be shorter than the latter {e.g., Platystoma). Its concavities 
may be subequal {Arius), but more frequently the posterior is much the deeper of 
the twm, and may even be almost tubular. One {Macrones) or two {Synodontis) pairs 
of nutrient foramina are present on its ventral surface, and apparently belong to the 
third and fourth vertebrae. A pair of accessory articular processes may be developed 
on the anterior margin of the ventral surface of the complex centrum, and may 
interlock with similar paired processes developed from the adjacent margins of the 
basioccipital and the centrum of the first vertebra {e.g., Macrones). Tiie centrum of 
the fifth vertebra is generally shorter than the complex centrum, hut in one or two 
instances {e.g., Platystoma) it may equal or exceed the latter in length. Its two 
concavities are rarely subequal ; more frequently the anterior is much the deeper, 
and sometimes may be even tubular {e.g., Aspredo, Platystoma). The sixth vertebral 
centrum is also nearly always larger than any that succeed it, although smaller than 
eitlier the complex or fifth centrum. Its concavities are equal or subequal, but 
occasionally, as in the fifth, the anterior may be much deeper than the posterior. 
The neural arch of the complex vertebra is always represented by two lateral and 
continuous plates of bone, confluent above with the spines of the third and fourth 
vertebrae and below with the centrum. The lateral vacuities, which in some Siluridoe 
{e.g., Platystoma and Auchenoglanis) partially interrupt the continuity of the arch, 
may possibly indicate a separation between the otherwise confluent arches of the 
third and fourth vertebras. As a rule, the arch is partially or completely anchylosed 
