MYODOME AND TRIGEMINO-FACIALIS CHAMBER 279 
fig. 27, pl. 31), and hence at the anterior end of the pituitary 
fossa of the adult. 
The pituitary veins, also, are not described by Sewertzoft, 
and neither they nor their foramina are indicated in his figures. 
They are, however, apparently shown by Baumgartner (’15) 
in sagittal sections through this region in embryos of Acanthias. 
In that author’s figures 2 to 9, he shows a vessel ventral to the 
anterior end of the notochord, and morphologically posterior 
to the hypophysis. This vessel is not lettered in the figures, 
but it must certainly be a cross-section of the venous commissure 
formed by the pituitary veins. In Baumgartner’s figure 9, it 
is shown lying between the parachordal plate above and a ventro- 
anteriorly directed process of cartilage that is apparently con- 
sidered by Baumgartner to be of parachordal origin, but which 
must represent a section through that part of the trabecular 
cartilage of Sewertzoff’s descriptions which is formed by the 
fusion of the trabeculae of opposite sides dorsal (posterior) to 
the hypophysis. This commissural vein would then pass dorsal 
to the trabeculae, as it normally should. The process shown 
by Baumgartner forms the posterior boundary of an opening 
between it and the hind end of the trabecular cartilage, and is 
hence the intertrabecular basal fontanelle of Sewertzoff’s descip- 
tions, and a vessel, possibly the internal carotid artery, is shown 
lying directly in it. 
The so-called intertrabecular basal fontanelle of these embryos 
of the Selachii would then seem to correspond to the fused anterior 
and posterior basicranial fenestrae of Salmo and Gasterosteus, 
the definitive fenestra of Pristiurus corresponding to the fenestra 
hypophyseos of Salmo and Gasterosteus, and the definitive 
fenestra of Acanthias corresponding to the fenestra basicranialis 
posterior of those fishes. This latter fenestra is, as will be later 
shown, the fenestra hypophyseos of the Dipnoi, Amphibia, and 
Sauropsida, in which the internal carotid arteries traverse the 
fenestra along its posterior border, sometimes separated by a 
median cartilage called the intertrabecula. 
Neither Sewertzoff nor Baumgartner describe polar cartilages 
in these fishes, but van Wijhe (’05) describes them in early 
