DEVELOPMENT OF THE SKULL IN STURGEONS. 
147 
This is the more remarkable in a type that might have been expected to show some¬ 
thing primitive in its visceral arches, seeing that it is only a Chondrosteus Ganoid, with 
a huge, permanently "undivided notochord, and a totally unossified chondrocranium. 
Even in the Salmon (Phi]. Trans., 1873, Plates 1-3), the embryos of which are much 
larger and more tractable, I did find, as cartilage, the primitive facial bars; and their 
metamorphosis into the highly modified hyostylic type could be traced step by step; 
but this cannot be done in Acipenser nor in Lepidosteus (my next subject). In 
Scyllium (Zool. Trans., vol. x., plate 34) this was also possible; but in the Skates, 
Raia and Pristiurus (ibid., plates 35 and 39) the earliest tracts of true hyaline 
cartilage were, already, half metamorphosed, for all the uppermost segments (ibid., 
plate 35, fig. 4) were developed as distinct cartilages, although the remainder of each 
arch was continuous, and its segmentation could be traced afterwards.'"' 
Here, in Acipenser , whilst the cartilage is still very soft, the segments of the highly 
subdivided hyoid arch are all apparent, although the branchial arches are continuous 
tracts (Plate 13, fig. 6); but their “ pharyngo-branchial 5 ’ segment—so very distinct in 
the embryo of the Skate—is already half severed from the main bar (Plate 13, fig. 4, 
p.br., e.br.). 
The pier of the mandibular arch, instead of growing up to the basis cranii, is a 
falcate cartilage (the pterygo-quadrate, pg.g), which lies half-way down the face, and 
passes forwards and downwards. It is flat, obliquely-placed, and somewhat uncinate 
in front; behind, it is thick and rounded, and forms a condyle which lies in the con¬ 
cavity of its own lower segment, the mandible (inn). This segment is thick behind; 
in front of its condyloid depression it sends upwards a coronoid swelling, whilst, 
behind, it ends in a rounded angular process. The rest of the bar is rounded; it 
lessens towards its ventral end, and is curved the opposite way to the pterygo-quadrate. 
The rounded quadrate region of the upper bar is attached by ligamentous fibres to 
the lower end of the hyoid pier; that pier is the hyomandibular (Jim.), the lower, 
rounded end of which is being segmented off as the symplectic (sy). The whole bar 
is a massive phalangiform cartilage, gently bent backwards, altogether running down¬ 
wards and a little forwards, and articulated above by an oblong condyle, to the 
c< tegmen tympani” of the ear-capsule, under the horizontal canal ( h.s.c .). It is one- 
third larger than the pterygo-quadrate, but its own lower piece, the cerato-hyal, is 
somewhat less than the mandibular pier. 
* In Selachians themselves, the best of all Fishes in which to study the development of these parts, it is 
next to impossible to maintain a consistent nomenclature of the elements of the visceral arches. In the 
Dog-fish ( Scyllium , ibid., plates 37, 38) the hyomandibular is evidently the serial homologue of the epi- 
branchials; in the Skate ( Pristiurus , ibid., plate 35) the uppermost part of all the arches is developed 
as a separate nucleus of cartilage, and the metapterygoid and hyomandibular naturally classify themselves 
with the succeeding pharyngo-branchials. Again, the subdivided hyoid pier of Acipenser, which also 
carries the mandibular apparatus, is not divided into a normal pharyngo- and epi-hyal, but the epi-hyal, 
apparently, is itself subdivided, and there is no pharyngo-hyal. 
u 2 
