552 
CAPTAIN E. W. SHANN ON 
The fibres from the outer line of insertion (c.m. l) run inwards, and those of 
either side eventually meet at a point (35 mm. Chimsera, 25 mm. Callorhynchus) 
behind the exposed portion of the mandible, thus hiding the fibres which run from 
the main line of insertion (c.m. 2) nearly parallel to the long axis of the body. 
After coalescing, the c.m. portions of either side unite and are only barely dis- 
tinguishable superficially, while internally their fibres become intermingled. The 
united c.m. portions gain in depth at the expense of their width, and the depth is 
further increased by the addition of fibres from the azygous (c.m. 2) portion. 
So far the two types under consideration are uniform, but from this point onwards 
they require separate consideration. In Chimsera the main mass of the c.m., derived 
from the entire c.m. 1 portion and from the superficial fibres of the c.m. 2 portion, 
is found to take origin from the deep groove on the anterior face of the coracoid 
symphysis. The remaining fibres of the c.m. 2 portion, shortly behind their insertion, 
become piled upon one another so as to form a vertical keel of muscle. This keel 
runs in a deep groove between the coraco-hyoideus muscles, from which it is 
separated by a fascia. At a point about 45 mm. behind their insertion the fibres 
which compose the keel are intercepted by a Y-shaped septum, whose apex is 
directed posteriorly. The deepest fibres of the c.m. may thus be described as arising 
from a wedge-shaped septum between the c.hy. muscles. Through a circular opening 
in either face of this septum passes the branch of the combined ventral roots of 
spinal nerves I and II which innervates the deep portion of the c.m. The above 
account differs from that of Vetter in that the superficial fibres of the c.m. 2 portion 
are described as running directly from their insertions on the mandible to join the 
deep fibres of the c.m. 1 portion near their origin, whereas Vetter describes the 
entire c.m. 2 portion as arising from the septum in question. Vetter, however, 
remarks that the c.m. 1 portion is augmented in the region of its origin by certain 
fibres from the septum, and my observations corroborate this. Briefly, the question 
at stake is : Are all the fibres, which arise from the anterior rim of .the coracoid and 
are inserted on the median (c.m. 2) area of the mandible, intercepted in their course 
by a septum ? Vetter’s answer is in the affirmative, mine in the negative. It is 
perfectly possible that there is variability in this respect, though my observations 
are drawn from an examination of four adult specimens ; but, in any case, the fact 
remains that the distinction between the two portions of the coraco-mandibularis 
(c.m. 1 and c.m. 2) is purely superficial. The condition recalls that observed in 
Rhina among Selachians. 
In Callorhynchus the c.m. runs without interruption throughout its depth from 
the mandible to the pectoral girdle ; the mass is characterised by its enormous bulk 
and by the peculiar distribution of its component fibres. The divisions of the c.m. in 
the area of insertion have been indicated above ; the superficial fibres of the c.m. 1 
portion, running towards the ventral middle line, soon cover the original ventral 
superficial fibres of the c.m. 2 portion, while their outer border is defined by a sharp 
