174 
PROFESSORS T. W. BRIDGE AND A, C. HADDON 
directly forwards and backwards respectively. The posterior face of each plate is 
faintly concave, the anterior very slightly convex. The protractor muscle of this 
“ elastic-spring ” mechanism has an origin and an insertion precisely similar to the 
corresponding muscle in Auchenipterus, and the effect of its contraction will be to 
pull the oval plate directly forwards towards the inferior limb of the post-temporal. 
The forward movement of each plate is confined within certain definite and com- 
paratively restricted limits by a projecting process of bone which grows out from the 
anterior surface of the elastic root and extends towards, but without quite reaching, 
a facet on the anterior margin of the arch of the complex vertebra, immediately in 
front of the foramen for the exit of the roots of the fourth spinal nerve (fig. 72, 
sp.n.^^). The forward movement of the plate on the contraction of its protractor 
muscle (pt.m.) will soon bring the process and the facet into direct contact, and any 
further movement in the same direction will be at once checked. The posterior 
division of the transverse process (t.p.^p.) is somewhat expanded but not decurved, 
and its root partially coalesces with the more slender transverse process of the fifth 
vertebra {t.p.^). Both processes have their ventral surfaces adapted to the convex 
dorsal side of the anterior compartment of the air-bladder. The transverse processes 
of the sixth vertebra are long and slender, with short ribs {r.') at their distal 
extremities. It is interesting to remark that, although each of the transverse 
processes of the fourth vertebra is modified to form an “ elastic-spring ” mechanism, 
it differs less from a normal process than is the case with either Oxydoras or 
Auc]icnip)terus, and in tliis respect more closely resembles the only other African 
Siluroid {Synodontis) in which this mechanism is known to exist. Not only does the 
process retain its characteristic anterior and posterior divisions, but the former more 
nearly resembles a normal and Inelastic anterior division in its relations to the 
anterior wall of the air-bladder. 
Superficial ossifications invest and greatly thicken the sides of the complex and 
fifth vertebral centra, their free ventral margins forming the lateral boundaries of a 
well-marked aortic groove. Anteriorly each ossification terminates in a lateral out- 
wardly projecting ridge coincident with the anterior margin of the complex centrum, 
and at the dorsal extremity of each ridge there is a small but distinct radial nodule 
suturally attached to the side of the centrum. From each nodule a slender splint- 
like dorsal lamina passes ventrad to the posterior cardinal vein, and eventually fuses 
with an oblique outwardly directed ridge on the ventral surface of the posterior 
division of the transverse process of the fourth vertebra, but for the remainder of its 
extent the channel for the cardinal vein is a deep groove between the thickened 
dorsal edge of each superficial ossification and the overlying roots of the transverse 
processes of the fourth and fifth vertebra). Posteriorly the superficial ossifications 
terminate in oblique thickenings, the ventral extremities of which are very similar to 
accessory articular processes, and overlap the anterior margin of the centrum of the 
sixth vertebra. 
