ON THE ANATOMY OF FISHES. 
157 
arches, but more especially by the superficial ossifications which continuously invest 
the lateral surfaces of their respective centra. The complex and fifth centra are about 
equal in length and width. The centra of the sixth and seventh vertebrae are much 
smaller, although larger than those of the free normal vertebrae which succeed them. 
The neural arch and short, thick spine of the third vertebra are both inclined 
forwards over the body of the first vertebra and partially anchylose with the exoccipi- 
tals and supraoccipital. The feeble spine of the fourth vertebra is directed obliquely 
backwards and supports the anterior interspinous bones of the dorsal fin, but is quite 
distinct from the preceding spinous processes. The remaining neural spines in the 
region of the dorsal fin, with the exception of the one belonging to the fifth vertebra 
are almost obsolete. The eighth vertebra is free and movable. 
By a singular modification, each of the transverse processes of the fourth vertebra 
becomes converted into the “ elastic-spring ” apparatus of Muller. The root of the 
process is a thin, highly elastic lamina of bone growing obliquely outwards from the 
neural arch and the backwardly inclined spine of its vertebra, and curving slightly 
downwards, with its anterior face looking obliquely upwards and forwards, and its 
posterior surface downwards as well as backwards (fig. 59, At its distal 
extremity the root expands into an antero-posteriorly flattened oval plate, composed 
for the most part of loose, cancellous and almost friable bone (fig. 59). As the result 
of the ventral flexure of the elastic root the two surfaces of the plate look inwards 
and outwards respectively, but the slight obliquity of the plate gives to the inner 
surface a tendency to look downwards and backwards, and to the outer surface an 
upward and forward inclination. The outer surface of each plate is somewhat 
convex, and the curious concentric and radial striations which traverse it are so 
related to a point on the dorsal margin, near its anterior extremity, as to render the 
surface very similar in appearance to the external face and umbone of an inequilateral 
Lamellibranch shell. The inner surface, on the contrary, is faintly concave and has 
only the radial striations. Between the free ventral margin of each plate and the 
commencement of the flexible root, there is a deep, but narrow, slit-like groove into 
which the thin outer edge of the crescentic process of the tripus is received (tr.c.). 
The inner surfaces of the two plates are closely applied to thin oval areas in the 
antero-lateral regions of the anterior compartment of the air-bladder. A specialized 
portion of the dorso-lateral musculature of the body wall becomes separated from the 
rest to form a strong protractor muscle for each of the oval plates. This muscle has 
its origin on the external surface of certain bones forming the posterior face of the 
skull, viz. : the posterior plate of the exoccipital and the epiotic, and from thence is 
continued downwards and backwards to its insertion into nearly the whole extent of 
the external surface of the oval plate (fig. 61, pt.m.). By the contraction of this 
muscle each plate may be pulled outwards and upwards, the elasticity of its 
flexible root allowing such movement to take place, but at once compelling the plate 
to return to its previous position when the muscle ceases to contract. The transverse 
