PLECTOGNATES. 
619 
have originally started from the dermal system, as for 
example the clavicles, here in many cases have the 
character of Ganoid scaly or scutate growths in the 
skin. This fairly prominent touch of resemblance to 
the Ganoids has induced scientists long to retain the 
majority of the following Physoclysts among the lowest, 
least modern Teleosts, th'ough the characters by which 
both the Plectognates and the Lophobranchii have been 
distinguished ever since the time of Cuvier, are evi- 
dently later modifications (higher metamorphosic degrees) 
of the Teleostean type. 
The Plectognates compose an order fairly rich in 
forms, varying both in the form of the body and, still 
more, in the texture of its covering. Some of them, 
most of the Balistoids, are of a fairly regular piscine 
form, others of a. polygonal form, which has given them 
the name of Coffer-fishes, others again, the Gymnodonts, 
of an elongated saccate or globular form. Some have 
hard, granulated scales, densely imbricated, juxtaposed 
to each other like a mosaic, or firmly united to a ca- 
rapace that may cover the greater part of the body, 
others small, spiny scales or, in their stead, loose, mo- 
bile spines, sometimes of considerable size, which are ! 
erected in self-defence. All the Plectognates have a 
comparatively small, but well-armed mouth, with strong, 
but few teeth, the strength of which is increased by 
the firm support afforded them by the coalescence and 
reduction of the component parts of the jaws. The 
short jaw bones form the tip of the facial part of the 
head, which part is generally considerably elongated 
and also deepened. This elongation chiefly affects the 
ethmoid bone, the extent of which on the upper surface 
of the cranium may rival that of the frontal bones, and 
the parasphenoid bone (the sphenoid bone of fishes), 
but is accompanied by the elongation of the preoper- 
culum and interoperculuinh The last-mentioned bone 
has a narrow, terete shaft, which extends from the hind 
inferior angle of the lower jaw along the inside of the 
lower anterior prong of the pr<?operculum, just as the 
latter extends back from the outer surface of the pro- 
cess Avith Avhich the loAver jaw articulates, on the qua- 
drate bone. The deepening of the facial part depends 
chiefly on the great vertical extension of the parasphe- 
noid bone and the pterygoid arch. Sometimes, it is 
true, as in the Balistoids, the palatine bone on each side 
may be a small, terete, and more or less mobile bone, 
bifid at the top and united to the ethmoid bone by 
ligaments. But in other cases, as in the genus Tetrodon , 
it resembles the pterygoid bone in form and is firmly 
united to the loAver surface of the ethmoid bone. Both 
the palatine bones and the vomer are toothless; and the 
pharyngeals, which are free, may, it is true, be armed 
with teeth, as in the Balistoids and Tetrodonts, but may 
also be destitute of teeth, as in the Coffer-fishes. The 
shoulder-girdle is generally Avell-developed. The post- 
temporal bone is indeed insignificant, being confluent 
Avith the mastoid bone, and the supraclavicular bone 
sIioavs scarcely anything unusual in its development; but 
the clavicular bone proper, though sometimes of ordi- 
nary form, may extend so far imvards in some of the 
Plectognates, as in the Coffer-fishes, that the bone of 
one side is contiguous to that of the other throughout 
the depth of the median plane of the abdominal cavity, 
thus forming a kind of diaphragm of bone Avith only a 
round, more or less tubular hole in the middle for the 
passage of the oesophagus. The shoulder-girdle of the 
Coffer-fishes is still further extended, beloAv the coracoid 
bones, by a pair of large ‘interclavicles,’ of Avhich in the 
preceding fishes Ave have found scarcely a trace, but 
which Ave shall again meet in the Sticklebacks in the form 
of Ganoid plates. In Batistes the clavicle extends so 
high up that Avith its upper extremity it touches the 
cranium, at the styloid bone, just within and under the 
articular surface of the supraclavicular bone. 
In Artedi the Plectognates formed the framework 
of the order which he called Brancliiostegi (i. e. with 
hidden gills , a reference to the narroAV gill-openings just 
above or in front of the insertions of the pectoral fins); 
and the principal character of the order, according to 
its founder, lay in the absence of special branchiostegal 
membranes. In general, however, the exact opposite is 
the case; the opercular apparatus is by no means im- 
perfect in these fishes. The operculum and suboper- 
culum are small, it is true, in the Balistoids and Coffer- 
fishes, but of normal size in the Tetrodonts. The ad- 
vanced development of the preoperculum has been 
mentioned above. The hyoid arches, to Avhich the 
branchiostegal membranes with their 4* — 6 rays are 
attached, have gained in breadth Avhat they have lost 
in length; and in most cases one of the rays in each 
membrane, usually the innermost, is considerably stronger 
than the others, sometimes, as in Tetrodon , several times 
a This elongation and deepening is most marked in the Scleroderms (see below); in the Gymnodonts it is less prominent. 
6 Sometimes 3, according to Bleeker. 
