THE SHOULDER GIRDLE AND PECTORAL FIN OF FISHES. 
563 
ft. Cli. br. interims. 
This muscle arises, often through the medium of a tendon (Trigla, Zeus, fig. 16), 
from the upper third of the cleithrum and follows an approximately horizontal course. 
It is usually considerably broader than the cli.br. ext. Hamburger states that in 
Lophius this muscle is inserted upon the hyo-branchialis muscle, but my dissections 
do not bear this out ; I find that the insertion occurs upon the fourth ceratobranchial 
in the normal manner ; the roots of the two muscles in question are, however, 
contiguous. 
(b iii). Cleithro-hyoideus. Sternohyoid, Stannius, Vogt and Yung ; 
Retractor hyoidei, Owen ; Hyo-pectoralis, M‘Murrich ; Hyo- 
clavicularis, Hamburger. 
A powerful muscle which forms the ventral border of the gill cavity. It has a 
double origin on the cleithrum ; the larger portion arises from the outer aspect of 
the cleithrum, usually in its lower third, while a smaller portion arises from the 
region of the clavicular symphysis, where it comes in contact with its fellow of the 
opposite side. The insertion occurs upon the base of the hyoid arch. The muscle 
is usually much reduced in bulk as it reaches the point of insertion, but Zeus (fig. 16) 
forms an exception. 
2. Pectoral Fin. 
(1) Adductor. 
The adductor system, as in the foregoing groups of fishes, is divided into two 
layers— -an outer, superficialis, and an inner, profundus. Each of these is divided 
distally into very distinct bundles which become inserted, usually by long tendons, 
upon the bases of the lepidotrichia. There is in addition in certain Teleosteans (Zeus, 
fig. 16) a third, deeper and more ventral, layer, which serves to dilate the fin. The 
third layer has been described by Cuvier, Yogt and Yung, and others, and is named 
the dilator posterior. These muscles take origin in part from the cleithrum and in 
part from the fin skeleton proper (coracoid, scapula, and radials). 
(2) Abductor. 
The abductor system conforms closely to the adductor in any given type of 
Teleostean. The third layer, when present, is named the dilator anterior. 
(3) Modifications. 
Extraordinary specialisation of the fin muscles occurs in forms whose pectoral 
fins have become modified for progression on a solid surface. These modifications 
have been fully described for Trigla by Williamson (92). 
The most interesting development of the muscles of the pectoral fin in the 
Teleostei is exhibited by Lophius. Hamburger (42) has given a complete account 
TRANS. ROY. SOC. EDIN., VOL. LII, PART III (NO. 21). 86 
