HYOMANDIBULA OF THE GNATHOSTOME FISHES 567 
ostomi, surrounds the nervus opticus, and the nervus opthalmi- 
cus profundus traverses it in its course through the orbit, thus 
in a measure being also surrounded by it. This sinus would 
thus seem to correspond to that stage in the development of the 
posterior sections of the vena capitis lateralis which Hochstetter 
(1. e., p. 183) describes as a ‘Veneninsel’ surrounding the related 
segmental nerve. A crescentic sinus is found, in the Plagios- 
tomi, similarly related to the nervus olfactoriuss 
These several variations in the relations of the vena jugularisto 
the cranial nerves, while they emphasize the facts that the Pla- 
giostomi form a group wholly apart from the other gnathostome 
fishes and that the Siluridae are similarly grouped apart from 
the other Teleostei, do not present any features which indicate 
that they have in any way influenced the development of the 
hyomandibula, as will be later evident. They can accordingly 
be neglected in the present discussion. ‘They however justify 
the use of the term vena jugularis rather than either of the terms 
vena cardinalis anterior or vena capitis lateralis, both of which 
are frequently used, and the terms vena petroso-lateralis or 
petrosa lateralis, introduced by Driiner (’01) for the correspond- 
ing vein in the Urodela and accepted by Kingsbury and Reed 
(09) as eminently appropriate for those animals, might, at present, 
be confusing if applied to fishes. 
In Chlamydoselachus the pharyngobranchials lie imbedded 
in what has, in one of three specimens examined, quite markedly 
the appearance of a continuous sheet of muscle fibers which is 
in process of differentiation into the Mm. interarcuales dorsales 
I of Vetter’s (74) descriptions of other selachians. This muscle- 
sheet lies immediately internal (dorsal) to the linmg membrane 
of the branchial cavity, extends from the hyal to the most pos- 
terior branchial arch, and, in the one specimen above referred 
to, its mesial edge lay everywhere slightly mesial to the dorso- 
mesial ends of the pharyngobranchials. The muscle fibers all 
run -postero-mesially, the muscle-sheet being considered, for con- 
venience of description, to run from in front posteriorly, and as 
the pharyngobranchials do not extend entirely across the sheet 
they have the appearance of being intercalated obliquely in the 
