THE SHOULDER GIRDL E AND PECTORAL PIN OF FISHES. 
535 
1892) that this division of the dorsal moiety into mesio-dorsal and latero-dorsal portions 
has no morphological value, I have found it sufficiently useful for descriptive purposes, 
and it is retained under this proviso. 
The ventral moiety in the anterior region is much modified owing to the presence 
of the abdominal cavity, whose walls it forms. The conical structure is almost lost, 
though, as we shall see later, evidence of it is retained in the lateral walls. The two 
portions described in the tail region are still recognisable externally by their myo- 
commata. Internally there is, however, a much closer relationship between the lower 
half of the C 4 section and the C 5 section than exists between the two halves of the 
C 4 section in the anterior region. This led Maurer to follow Humphry in his 
designation of the latero-ventral (L.V.) and mesio-ventral (M.V.) portions of the 
ventral moiety, the division being indicated in text-fig. 1. It is seen, then, that the 
latero-ventral portion comprises the upper half of the C 4 section, while the mesio- 
ventral portion comprises the lower half of the C 4 section together with the entire 
C 5 section. 
In the latero-ventral portion the fibres throughout their depth run approximately 
parallel to the long axis of the body, the ventral limit of the muscle being defined 
externally by the backward slope of the myocommata, and internally by the distal 
extremities of the ribs. In the mesio-ventral portion the fibres run obliquely from 
downwards anteriorly to upwards posteriorly, but their obliquity gradually disappears 
as the ventral surface is peached, till they come to lie parallel to the long axis of the 
body, in which condition they reach the ventral septum which separates them from 
their fellows of the opposite side. These two superficial areas of the mesio-ventral 
portion were declared by Maurer (ibid.) to be the homologues of the obliquus 
internus and rectus respectively of the Amphibia. Moreover, on dissecting away the 
superficial oblique fibres, Maurer observed a second layer of oblique fibres, which 
took the opposite direction (namely, from upwards anteriorly to downwards posteriorly), 
and this layer he took to represent the transversus of the Amphibia. Humphry also 
noted this stratification of the mesio-ventral portion. There is, however, no septum 
between these two layers ; indeed, they pass by gradual transition one into the 
other. I have given in detail elsewhere * my reasons for regarding this pseudo- 
stratification of the lateral muscle in the anterior region as a result of the conical 
conformation that occurs posteriorly. 
(2) The Posterior Muscles. 
The lateral muscle when it reaches the pectoral girdle becomes interrupted in its 
course by that structure, either throughout its depth or in the superficial region only. 
The portions of the lateral muscle which are thus attached to the girdle constitute 
its posterior system of muscles, and, from the nature of their position, act as retractors. 
Proc . Zool . Soc ., 1914, pp. 319 et sqq . 
