1912. ] R. B. S. SEweLL: Notes on deep-sea fish. 5 
The mouth -is large and has broken through, and is 
bounded posteriorly by a complete lower jaw, the two mandibles 
having united in the middle line, and the point of union being 
marked by a rounded backwardly directed projection. 
The eyes are large and bulge prominently on each side of 
the head; their maximum diameter is 1°5 mm. and the in- 
terocular diameter of the head 3 mm. Below the eye, between 
it and the mouth, is a small and not very distinct depression, 
corresponding in position almost exactly with the aperture des- 
cribed by Dean (i.c., p. 104, pl. viii, fig. 49 d) in Chimaera as 
the rudimentary spiracle—a most unusual situation for it, if this 
view be correct. 
Balfour in his classical work on the development of the Elas- 
mobranchs (Humphry and Turner’s Journal of Anatomy and 
Physiology, vol. x, pl. xxv, M and M’) has figured a young embryo 
of Pristiurus, which shows a well-marked triangular depression 
between the eye and the mouth in a situation precisely similar to 
Bashford Deans “‘spiracle.’’ (The true spiracle is shown to be 
present, much more posteriorly, between the eye and the Ist gill- 
cleft.; This depression, the morphological significance of which 
seems to have been erroneously interpreted by Bashford Dean, is 
apparently formed by the forward extension of the Ist visceral or 
mandibular arch on either side of the primitive mouth. 
McIntosh and Prince (1890, p. 751) have shown. that, in 
Teleosteans, the mandibular arch, though originally single, subse- 
quently splits into two, an anterior palato-quadrate portion and a 
posterior true mandibular part. The former, apparently, at a some- 
what later date extends forwards below the eye on either side of 
the mouth and finally unites with the olfactory region of the snout. 
Judging from Bailfour’s figures and description (/.c., p. 563) a 
precisely similar process occurs in Pristiurus. 
What happens in Chimaera (and in all probability in Rhino- 
chimaera also) is, so far as one can tell, that this palato-quadrate 
portion of the mandibular arch grows forwards and, when it 
reaches the lateral part of the olfactory region, fuses with it, 
leaving, as Bashford Dean has shown, an aperture between the 
mouth and the exterior just below the eye, ina position exactly 
analogous to the lachrymal duct in higher vertebrates. This canal 
subsequently disappears, only a trace of it being visible, as a 
shallow depression, in embryos that have reached a stage of 
development corresponding to Balfour’s stage M in the shark 
(Balfour, /.c., vol. x, pl. xxv). It is obvious from Bashford Dean’s 
figures that his so-called ‘‘spiracle’’ lies not only in front of the 
mandible but even anterior to the palato-quadrate or maxillary 
offshoot from this arch and hence cannot possibly be the true 
homologue of the spiracle, 7.e., a gill-cleft immediately anterior to 
the ist gill-arch between it and the mandibular arch. 
The Gills. Five gill-arches are present and project markedly 
at the side of the head; they are fringed with long external gill- 
filaments in which the vein and artery can be distinctly made out 
