Br. R. H, Tmquair — On Chondrosteus acipenseroides. 253 



shape, witli the suboperculum of Palcsomscus ; ^ and that this is its 

 true interpretation is proved by the discovery of the proper oper- 

 culum (op.) lying above it and between it and the cranial shield 

 (Figs, 2, 3, 5). The shape of this operculum may be aptly likened 

 to that of an inverted comma, the tail passing upwards and forwards 

 to the cranial shield, the convex margin being posterior and the 

 concave one anterior, a considerable space in front of the bone and 

 above the suboperculum being still left uncovered. The opercular 

 flap is continued downwards and forwards by a series of ten 

 imbricating brancJiiostegah (br,), which are broad and plate-like 

 where they immediately follow the suboperculum, though anteriorly 

 they become narrow and slender. 



Jaws and palato-quadrate apparatus. — The maxilla (mx.) is a 

 tolerably stout bone, tapering anteriorly and somewhat expanded in 

 its posterior third — the posterior margin looking very obliquely 

 upwards and backwards. As is well shown in Figures 2 and 8, it 

 is curved inwards in front to meet its fellow of the opposite side in 

 a perfect symphysis ; there cannot, therefore, be a true praemaxilla 

 here any more than in Acipenser or Polyodon. Articulated to the 

 oblique posterior mai'gin of the maxilla is a small flat plate (j), whose 

 shape somewhat reminds one of a boot, the sole being in apposition 

 with the maxilla, while the leg is directed upwards and backwards 

 towards the anterior margin of the suboperculum (see Fig. 4, j). 

 This is clearly the homologue of the little bone which in the recent 

 Sturgeon is called " prseoperculum " by Mr. W. K. Parker,^ although 

 it seems to me and others to be the same "jugal" element which 

 we find appended to the maxilla in Amia, Salmo, etc. In the Sturgeon 

 it has also been called " maxilla " by some who looked upon the real 

 maxilla as a " preemaxilla," ^ and this seems to have been Sir P. 

 Egerton's view when he assigns a preemaxilla as well as a maxilla to 

 Chondrosteus. But an examination of the original specimen repre- 

 sented in pi. Ixix. of his memoir shows that his " prasmaxilla" is the 

 suborbital bone, and that his maxilla — the small bifurcate bone 

 behind it — appertains to the palate, and will be described immediately 

 as a pterygoid element ! The real maxilla in this specimen is inter- 

 preted by Sir Philip as a mandible, while to this jugal plate he has 

 assigned the name and position of hypotympanic (=quadrate). 



Within the space bounded by the maxillce, the roof of the mouth is 

 principally composed of two plates (m. pt. Figs. 2 and 4) of a some- 

 what oval or ovoid contour, narrower behind than in front. Ante- 

 riorly, these plates are placed behind the symphysis of the maxillee ; 

 mesially, they articulate with each other along a portion of their 

 internal margins, while externally each comes into contact with 



_ 1 Described as " interoperculum " in my Memoir on the Structure of the Palaao- 

 niscidae. I have, however, abandoned that view, and now consider the plate inter- 

 calated between it and the opercidum in such genera as Rhabchlepis to be not a sub- 

 operculum, but merely an accessory element. 



2 " On the Structure and Development of the Skull in Stm-geons," Phil. Trans, 

 vol. 173 (1881), p. 172. 



3 See Stannius, " Handbuch der Zootomie der "Wii'belthiere, " erster Theil, Die 

 Fische, p. 53. 



