826 
SCANDINAVIAN FISHES. 
THRISSOMORPHI. 
Pliysostoms with the shoulder-girdle suspended from the head (as usual in the Teleosts). Scapular disk inter- 
nally strengthened by an arch formed by a special bone ( os prcecoracoideum) from the coracoid bone to the clavicle. 
The first four abdominal vertebrae of normal form and development. Hyomandibular and pterygopalatine arches 
complete, as well as ( in most cases) the opercular apparatus. Maxillary bones fully developed. 
That the Salmons and Herrings have much in 
common, was declared by Agassiz in 1843", when he 
united them into one family, Halecoides; and Cope 
followed up this opinion in 1871* by the establishment 
of the order Isospondyli, answering to the series of the 
Thrissomorphs, and distinguished from the preceding 
series mainly by a negative character, the absence of 
the so-called acoustic bones. The connexion between 
the organs of hearing and the air-bladder, however, is 
not foreign to this series. In the Clupeoids Weber 0 
discovered that the air-bladder, which tapers forward 
for some length, is divided in front into two branches, 
one to the right and one to the left, which pass through 
the occipital bone into the cranium, and there, as Hasse^ 
has shown, touch with their tips the outer wall of the 
lymphatic chamber surrounding the auditory apparatus. 
Sagemehl has given good reason 0 for his assumption 
that this mode of communication is the more primitive, 
from which the more complicated connexion of the 
preceding series has been evolved, and which has been 
persistent not only in the Clupeoids and some other 
Pliysostoms, but also in some Physoclysts of ancient 
type, as for instance the Berycoids (cf. above, p. 66), 
the Macruroids, some Gadoids and Balistoids. We have 
also seen a reminiscence of this connexion in the two 
tubular processes which ascend from the anterior ex- 
tremity of the air-bladder of the Gadoids (cf. above, 
p. 476) in vermicular curves towards the head, one 
on each side of the anterior renal mass. The absence 
of the so-called acoustic bones is thus explained as an 
earlier stage of development; and a negative character 
of this signification can hardly be sufficient to define 
a natural evolutionary series. We also find that a great 
number of the Thrissomorphs are without the character 
adduced by Agassiz as one of the most important in 
his definition of Halecoides, namely the participation 
of the maxillary bones in the formation of the margin 
of the upper jaw. Thus we might reasonably treat 
these forms as a distinct series of families ( Scopelo - 
morphi); but the resemblances in other respects are 
sufficiently great to render the Thrissomorphs a natural 
whole, with the same limitations as Cope’s Isospondyli, 
although the said resemblances do not constitute cha- 
racters applicable to the Avhole series. Within this 
series too the variation of form and structure is great 
and affects most of the organs. Scales and a distinct 
lateral line may be present or wanting. The maxil- 
laries may be simple, as in the rest of the Teleosts, 
or composite, furnished with so-called supplementary 
bones answering to the cheek-bones ( ossa jugalia ) of 
the higher vertebrates. The teeth may be numerous 
and well developed or wanting. A so-called adipose 
fin is present on the hind part of the back in many 
forms, but wanting in others. Luminous organs, so- 
called phosphorescent spots, may be present or absent. 
The ovaries may be furnished with an oviduct or with- 
out a complete tube of this description. The air-bladder 
may be present or wanting. Such great differences in 
forms which nevertheless show their natural affinity 
in some way or other 7 , render the characterization of 
° Recti. Poiss. Foss., tom. V, pp. 3 and 96. 
6 Iclitliyol. Less. Ant., Trans. Amer. Philos. Soc., Philad. N. ser., vol. XIV, pp. 452 and 454. 
c De anre cett., p. 73, cett. ; figg. 6.3, cett. 
d A.nat. Stucl., Bd. I, p. 599, cett. 
e Morpliol. Jahrb., Bd. X, p. 51. 
f Thus for example the Haplochitoiridce — - a family of fresh-water fishes from South America and Australia — are almost typical 
Salmons, but have the margin of the upper jaw formed by the intermaxillaries alone; and similarly the Sternoptycliidce and Scopelidce are 
families so like each other that they ought hardly to be kept apart, though in the former the osseous framework of the mouth follows the 
Halecoid type, while the latter preserve in this respect the ordinary Teleosteous character. 
