THE SHOULDER OIRDLE AND PECTORAL FIN OF FISHES. 
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ridge, above which is a deep trough, gouged, as it were, from the lateral aspect of the 
muscle. This ridge and trough are most marked anteriorly, and gradually disappear 
posteriorly, the ridge being rounded off and the trough filled out ; this effect is 
attained at a quarter of the length of the muscle from its origin on the coracoid. 
The deeper fibres of the c.m. 1. portion, instead of making for the middle line, sweep 
outwards and upwards, forming the floor of the trough which was mentioned above, 
and find origin on the anterior face of the girdle. The origin of this portion is, in its 
ventral area, exposed on the lateral aspect (after removal of the gills and operculum) ; 
dorsally, however, it is covered by the coraco-branchiales, beneath which it extends 
upwards to the scapular region of the girdle, slightly above the level of the adductor 
muscles of the fin. The c.m. 2 portion is not interrupted, as in Chimsera, by a septum, 
but extends as a solid mass of muscle within the outer coating of the c.m. I portion. 
The mesial fibres of the c.m. 2 portion blend with the mesial fibres of the c.m. 1 
portion, where the faces of these muscles are in contact, and accompany them to their 
origin. The lateral fibres sweep outwards and upwards, running beneath and parallel 
to the lateral fibres of the c.m. 1 portion. This powerful mass of muscle, whjtfh 
represents the fused coraco-mandibulares of either side, is, then, near its origin on the 
pectoral girdle, a homogeneous structure, whose section is U-shaped, the concavity 
being directed dorsally. 
2. Pectoral Fin. 
The morphology of the pectoral fin skeleton of the Chimseroids has been discussed 
by Howes (48, p. 22). In this discussion, Mivart’s interpretation of the relationship 
of the parts is advocated, and subsequent authors (e.g. Goodrich, Lankesters Treatise 
Zook, ix, p. 81, Oallorhynchus) appear to accept this view. The fin is supported 
upon two basal elements, both of which articulate with the glenoid border of the 
pectoral girdle ; the postaxial is held to represent the metapterygium, the preaxial the 
proptergyium (the mesopterygium is wanting). A diagnostic triangular cartilage, 
whose base articulates proximally with the propterygium, represents the fused 
portions of the four propterygial radiales ; this will be designated in brief the 
propterygial radial. 
(l) Adductor Musculature. 
The adductor musculature closely resembles that of Sharks ; that is to say, a series 
of muscle-bundles radiate fanwise from the coraco-scapular region of the girdle to the 
blade of the fin. The two preaxial bundles, inserted on the propterygium and 
propterygial radial respectively, are specialised and will receive separate treatment. 
The remaining thirty-one bundles are divided, though indistinctly as in Selachians, 
into a superficial and a deep layer. Davidoff (20) recognises a layering in the 
adductor musculature of the pelvic fin of Chimsera. 
The following account is based on a study of China sera alone, a certain striking 
point of difference from Oallorhynchus being noted in passing. 
