THE SHOULDER GIRDLE AND PECTORAL FIN OF FISHES. 
545 
2. Pectoral Fin. 
(l) Adductor {dorsal or inner) Musculature. 
The adductor muscle of the fin is a fan-shaped structure, composed of a number of 
muscle bundles which radiate from the axil to the blade. With the exception of a 
few of the outermost (the first three in Scyllium), each bundle is divided horizontally 
into two portions, so that the whole muscle appears to be divided into two super- 
imposed layers. Most authors recognise but one layer in this muscle; Davidoff (20), 
who studied very thoroughly the musculature of the pelvic fin of Elasmobranchs, 
distinctly states that in that region there are two layers in Selachians (Acanthias). 
The evidence which I have been able to collect from the large amount of embryo- 
logical work published on this subject militates against the view that the division 
of the muscle into superimposed layers is of real morphological value. Balfour’s 
figure of the development of the pectoral fin of Scyllium (81, pi. xviii, fig. 7) 
certainly appears to show two layers in the adductor muscles. This may, however, 
only be an apparent layering ; the “radial muscles” ( = adductor bundles) are admit- 
tedly oblique in older embryos (such as that figured), and this involves a certain 
amount of overlapping of adjacent radial muscles ; thus, the segments appearing in 
the section may represent parts of different muscles, not different parts of the same 
muscle. That two layers exist in the adult, at all events in the proximal region, 
there can be no doubt ; the outer layer is attached to the girdle, the inner to the 
proximal cartilages of the fin, and between the two a well-marked cavity is found. 
Proceeding outwards the layers approximate to one another, and finally coalesce so 
closely that it becomes difficult to separate them. The muscle will be described 
here in terms of two layers, namely, adductor superficialis (add.s.) and adductor 
profundus (add.p.). 
(i) Adductor superficialis (add.s.). 
Arises from the scapula and is inserted into the adductor profundus. In Scyllium 
the three preaxial bundles are not divided ; that is to say, they may be regarded as 
belonging entirely to the superficial layer, since they arise on the scapula. These 
three bundles are inserted on the propterygium and propterygial radial. The 
remainder of the add.s. is divided into two regions, an anterior and a posterior. 
Near their origins on the scapula the anterior (preaxial) portion passes beneath the 
posterior, and its innermost fibres become attached to the inner border of the scapula. 
This arrangement of the superficial muscles allows a certain degree of rotation, or 
curling, of the fin. 
The general arrangement of the superficial layer, with regard, to origin and inser- 
tion, holds good in all the types under examination ; but the division into two 
overlapping portions is only found in the shark-like forms, while in Rhina and Raia 
the muscle bundles radiate in perfectly regular sequence. 
