DEVELOPMENT OE THE SKULL IN STURGEONS. 
153 
newer cartilage ( mt.pg'.) has made its appearance since the last stage; this is, as far as 
I know, an unique structure, and but for this stage I should have suspected that this 
keystone piece had been formed by the fusion of a right and left nucleus of cartilage. 
Others form round it afterwards, but this is the first, as it is also the most important, 
morphologically, of the pieces that form the hind palate of the Sturgeon. 
It would not have been difficult to have dealt with the paired pieces that appear 
afterwards (Plate 16, figs. 5 and 6), as they are manifestly the counterparts of the 
single or multiple “ metapterygoid” cartilages of the Rays. In Torpedo (Gegexbaur’s 
‘ Selachians/ plate 13, fig. 3, and plate 20, fig. 1, Kr., a, b ,*) there are four of these 
cartilages on each side, but as a rule there is only one in that group, where it is a 
true metapterygoid segment of the pier, and not a mere “ ray,” as in the Shark. In 
Carinate Birds, ■where the skeleton reaches the utmost limits of specialization, remnants 
of Ichthyopsidan palatal cartilages reappear—ethmo-palatine, post-palatine, &c. ; and 
in the Woodpeckers (“Picidse”) the palatine membrane bones are joined together under 
the basis cranii by an ossified cartilage, the medio-palatine (Trans. Linn. Soc., second 
series, “ Zoology,” vol. i., plates 1-5).* 
Muller simply calls the whole compound plate of the adult Acipenser ruthenus 
(op. cit., plate 9, fig. 71, A, B) “ unpaariger Gaumenknorpel,” and the pterygo- 
quadrate bars “paariger Gaumenknorpel,” so that the interpretation of these parts is 
left open. I shall merely, for the present, classify this plate with the free cartilages 
of the “ Raiidae,” and call the median piece the “ azygous metapterygoid ” (mt.pg.) ; 
the later pieces, right and left, will simply be called “ paired metapterygoid segments ” 
(mt.pg".). In the adult (Plate 18, fig. 4) Mr. Howes finds a row of four more much 
smaller segments along the mid line, in front of the main piece.f 
The mandibles (mn. } mk.) are about as long as the upper bars, but after forming a 
shallow articular cavity on their thick upper end they become rounded rods, and their 
position is almost transverse, as these dissections, and the transverse sections (Plate 15, 
fig. 3, mk.) demonstrate; they do not quite meet in the middle, at present. 
The hyoid arch has five segments in it on each side, but, like the mandible, no basal 
segment below. The uppermost piece, the hyomandibular (Plate 13, figs. 11, 12, hm.) 
is by far the largest element in the whole series of arches ; above, it is articulated by 
an oval condyle to the under surface of the pterotic ridge, under the horizontal canal 
(“ tegmen tympani”), and below by a cylindroidal condyle to the symplectic (sy.). 
* The median cartilage seen in the “Myxinoids” (Muller, Myxinoids,” plate 3, figs. 2-5, U), the 
Gaumenplatte which partially unites with the trabecula) in the adult Bdellostoma and Plyxine, and 
early and totally in P etromyzon, is not part of the palate, as Muller’s term would suggest, and as I once 
thought, but is, as Mr. Ealfour showed me, the “ intertrabecula ”— i.e., its hinder or interorbital part. 
f In the scanty living remnants of the “ Chondrosteous Ganoids,” only the “ Acipenseridae ” show this 
peculiar structure; the “ Polyodontidas ” show no brace of it (see Muller’s ‘Myxinoids,’ plate 5, fig. 7, 
there called Planirostra edentula; Traquair, ‘ Ganoid/Eishes of the British Carboniferous Formations,’ 
Pait I., Palgeoniscidas, Palsnont. Soc., 18/7, plate 7, fig. 1; and Bridge, rl On Polyodon folium,” Phil, 
Trans., 1878, Part II., Plates 55-57, pp. 683-733. 
MPCCCLXX XII. x 
