HEMIBRANCHS. 
635 
PHYSOCLYSTI HEMIBRANCHII. 
Physoclysts with free jawbones and pectinated gills, but with the branchial arches more or less imperfect and 
with large and externally visible inter clavicles. 
Here, as in the case of the Plectognates, the sy- 
stematic difference from the other Physoclysts is not at 
all too great to admit of the arrangement of these fishes 
in a series of families corresponding to the series already 
adopted among the Physoclysts and ranged beside these 
series. This was also the approximate rank assigned to 
them by the writer who first treated of the combination 
of these fishes into a systematic whole: in 1861, in his 
arrangement of the Acanthopterygians", Gunther adopted 
a twelfth division, Acanthopterygii g aster ostei formes, in- 
cluding the Sticklebacks and Flute-mouths with their 
nearest relatives, and a thirteenth division, Acanthopte- 
rygii centriscif ormes , containing the family of the Trum- 
pet-fishes. After Parker’s * 6 demonstration in 1868 of 
the morphological significance of the interclavicles of 
the Sticklebacks as traces of the Ganoid type, Cope 0 
united Gunther’s two divisions into an order Hemi- 
branchii, “connecting the Lophobranchii with ordinary 
fishes.” 
All these fishes have abdominal ventral fins, and 
are thus ranged comparatively low in the scale of deve- 
lopment of the Physoclysts. Their principal character 
also lies, as we have shown above, in the circumstance 
that several of the bones which in more typical Phy- 
soclysts entirely partake in the structure of the endo- 
skeleton, are here wholly or partly dermal, appearing 
in the form of growths belonging to the exoskeleton. 
This structural feature is fairly common among fishes. 
Scales, differing usually in size, shape, or thickness from 
the other scales of the body, are in one fish the pre- 
cursors of a still further alteration in a kindred form, 
a difference which, during the development of the form- 
series, leads to the result that the modified scales creep, 
so to speak, into the body and join themselves to parts 
of the endoskeleton, and finally, when the transforma- 
tion has reached its highest point, lie like covering 
(membrane) bones outside other bones, or even enter in 
the form of independent bones into the endoskeleton. 
A great portion of the skeleton of the Vertebrates has 
originated in this manner. The most beautiful and most 
perfect example of this is given by the human clavicles, 
the development of which has been traced by Parker 
(1. c.) from their first origin, at which period they are 
morphologically analogous to two lateral plates in the 
Sturgeons or the Cuirassed Siluroidsk The intercla- 
vicles which we have above remarked in certain Plecto- 
gnates, are of similar origin. They occur, it is true, 
in several other Physoclysts as well, but in an extre- 
mely reduced form and as parts of the endoskeleton. 
Here, in the Hemibranchii, as well as in the Lopho- 
branchii, they still preserve distinctly the character of 
dermal plates. Thus, in the Three-spined Stickleback 
(fig. 156, C) for example, they have the form of two 
parallel, ventral scutes, contiguous in front, separated 
behind, on the outside granulated and striated, like the 
other dermal plates of the body, on the inside support- 
ing, in front, the lower ends of the clavicles, and be- 
hind, the loAver margins of the coracoid bones or more 
correctly, according to Parker, of their covering bones, 
the procoracoids, which are also said to be of dermal 
origin. 
r fhe same relative position to the rest of the 
skeleton is here occupied by the pelvic bones or rather, 
according to the same interpretation, by the covering 
bones of the pelvic bones proper. These bones lie in 
the Sticklebacks immediately behind the interclavicles, 
or even project some distance between them; and in the 
Three-spined Stickleback (fig. 157, A, vs) they are 
marked by the large, ascending process that meets the 
lateral plates of the body in the skin. The character 
which in Cope has been chosen to give the Hemibranchii 
their name, lies in the absence or persistent cartilagi- 
nous structure of one or more of the upper parts of 
the branchial arches (the epibranchial and upper pha- 
“ System. Synops. Fam. Acanthopt. Fish., Cat. Brit. Mus., Fish., p. VIII. 
6 Ray. Soc., Shoulder Girdle and Sternum, p. 40. 
c Ichthyology of the Lesser Antilles, Trans. Airier. Phil. Soc., Philad., n. ser., vol. XIV, pp. 456 et 457. 
d Cf. Smitt, Ur de hogre djurens utvecklingshistoria, p. 218. 
