Contribution to Knowledge of Paleeospondylus Gunni. 315 
fontanelle, comes into relation with the pharynx, into which 
it actually opens in the former, while in the latter it 
ends in a blind sac. But as there is no_basicranial 
fontanelle in Paleospondylus, there could have been no 
posterior nasal canal; so that if we are dealing with a 
Marsipobranch, we have here a condition which is either 
more archaic or more specialised than that in the modern 
representatives of the group, according as in this particular 
we consider Myxine or Petromyzon to be the more primitive." 
Behind the median cirrated opening, the anterior or 
trabeculo-palatine moiety of the cranium (¢.p.) is almost 
invariably so eroded or broken through that the appearances 
presented would scarcely be readable without the tolerably 
perfect specimen represented in my former paper (5, Pl. L., 
Fig. 2), and refigured, more highly magnified, in the present 
Pl. [X., Fig. 5. Here and in the restored drawing (Fig. 8) it 
will be seen that the surface external to the median longi- 
tudinal groove is divided by a line passing obliquely back- 
wards and inwards from the external angle, and marking off 
a small lobe (a.), which can also constantly be recognised in 
eroded specimens (Figs. 2, 3).2 In front of this line the 
surface shows two depressions like fenestre (0.,c.), but perhaps 
not completely perforated, each of a somewhat triangular 
shape, the apices of the two converging outwards towards the 
external angle, their rounded bases directed inwards. The 
division between these two apparent fenestre also constantly 
appears in eroded heads, as a bar directed obliquely forwards 
and inwards, as is seen in Pl. IX., Figs. 1, 2, and 4. 
The bottom of the median groove (intertrabecular space) is 
not perforated, but solid, and is seen isolated, as an antero- 
posterior median bar, in Figs. 1, 2, and 3, the raised edges 
(trabeculz) being gone. But in Fig. 4 it is itself gone, while 
1 If the nasal opening and canal of Myaxine represent the primitive verte- 
brate mouth, as is maintained by Beard (Anat. Anzeiger, 1888, No. 1, 
pp. 15-24), then of course the condition in that genus is more primitive than 
in Petromyzon, and in both than in Pal«ospondylus. But morphologists do 
not seem to be agreed as to which of the two recent Marsipobranch types is 
the more archaic. (See Howes, 3, pp. 137, 138.) 
2 It may be fancy, but I cannot help in my mind comparing this lobe 
with the ‘‘styliform” or ‘‘epihyal” element of the skull of the Lamprey. 
