590 ALLIS. [VoL. XII. 
interspaces between them and then to their inner surfaces, 
dragging the nerve along with them. It seems much simpler 
to suppose that the branchiostegal rays and gill-rakers are not 
homologous structures, and that the former have been devel- 
oped in their present position as simple dermal bones. 
e. Adductor Hyomandibularis, Adductor Operculi, and Levator 
Operculi. 
These three muscles arise from the muscle Csd@2 of Vetter, 
that is, from a muscle which he derives from the dorsal half of 
the superficial constrictor of the hyoid arch. Csdz arises in its 
anterior portion, in Heptanchus, Acanthias, and Scymnus, from 
the upper, outer corner of the occipital region of the skull. In 
its posterior portion it arises in Heptanchus mainly from the 
lateral edge of a superficial dorsal fascia, and in Acanthias and 
Scymnus from the edge of a corresponding fascia, and from a 
tendinous streak which marks the place where the long free 
edge of the first gill-cover or wall in Heptanchus has in part 
disappeared by fusion with the next following partition (No. 124, 
Note 1, p. 414). In Acanthias this fusion incloses and marks 
the position of a part of the lateral canal of the head. 
The posterior part of the muscle in all three fishes is either 
directly continuous with the ventral half of the constrictor, or 
is inserted with that muscle in an aponeurotic formation which 
lies between them. The anterior part of the muscle is, in 
Heptanchus, inserted mainly along the hind, upper edge of the 
upper jaw, a thin layer only, which separates from the inner 
surface of the muscle, beingyinserted on the outer edge of the 
upper member of the hyoid arch. In Acanthias the muscle is 
inserted mainly on the front, lateral edge of the hyomandibular, 
a small portion only, forming the anterior bundle of the muscle, 
being inserted on the upper, hind corner of the upper jaw. In 
Scymnus even this slight attachment to the upper jaw is not 
found, the anterior fibres of the muscle, corresponding to 
the anterior bundle in Acanthias, being inserted instead on a 
strong ligamentous band which extends from the upper edge 
of the hyomandibular to a projecting corner of the hind edge 
of the upper jaw. 
