. ON THE ANATOIklY OF FISHES. 
1G9 
anchylosis of their neural arches and spines, and also with the skull by the firm 
sutural union of the arch and spine of the third vertebra with the supraoccipital and 
the exoccipitals, and by a similar union of the transverse processes of the fourth 
vertebra with the post-temporals. The body of the first vertebra (fig. 70, y.^) is 
unusually small, being represented by a thin discoidal bone firmly wedged in between 
the basioccipital and the complex centrum. Its ventral surface is visible externally, 
but dorsally the centrum is partially excluded from the floor of the neural canal by 
the conjoined margins of the complex centrum and basioccipital. The complex 
centrum (c.c.) is more than twice the length of the centrum of the fifth vertebra 
which in turn is but a little larger than one of the succeeding free normal centra. 
The sixth vertebra is normal and freely movable on the foregoing. Owing to tlie 
width of the intervertebral ligament between the fifth and sixth centra, and the 
complete freedom of the arch and spine of the latter, there is considerable lateral 
and vertical mobility betw^een the sixth and remaining trunk vertebrae and the fifth 
and preceding vertebrae. 
The spinous processes of the third, fourth, and fifth vertebrae (fig. G8, 7i.s.^, n.s.^, 
and n.s.^) are confluent, and together form a high, laterally compressed lamina of bone, 
with an oblique backwardly projecting posterior margin, a dorsal edge which is some- 
what rounded behind but slopes downwards in front, and a relatively short anterior 
margin. The anterior portion of the plate, representing the spine of the third 
vertebra, is inclined forwards and, with the neural arch with w^hich it is continuous 
inferiorly, firmly articulates with the supraoccipital and exoccipitals, and also along 
its dorsal border with a descending process derived from the supraoccipital spine 
{s.o}). The posterior section of the lamina, representing the spines of the fourth and 
fifth vertebrae, and separated from the anterior by a well marked semicircular notch 
in the dorsal margin, is longitudinally cleft to its root into two thin opposed plates"' 
separated by an interval sufficiently wide to receive the modified first and second 
interspinous bones of the dorsal fin (^.s.,^ and The inner surface of each of the 
two opposed plates is transversed by a series of closely set, vertically arranged and 
parallel ridges while the first interspinous bone is similarly ridged on both its surfaces 
like a double file. The vertical flexure of the sixth and succeeding vertebrae on the 
anterior rigidly interconnected vertebrae will necessarily cause the ridges on the 
interspinous bone to scrape against those on the inner surfaces of the cleft neural 
spines, and therefore probably lead to the production of more or less acute voluntary 
sounds. 
The transverse processes of the fourth and fifth vertebrae combine on each side to 
form a hollow, transversely disposed and somewhat shallow bony funnel, the mouth 
of which is slightly expanded and closed only by the lateral cutaneous area of its 
* In the paper on the stridulating mechanism referred to above the two plates are said to bo formed 
by the vertical division of the neural spines of the coalesced anterior vertebrae, but more precisely, and 
as here stated, they represent the confluent and cleft spines of the fourth and fifth vertebrae. 
MDCCCXCIII. — B. Z 
