I)v E. IT. Traquair on Chonclrosteus acii^enseroides. 355 



plate {s.op., Figs. 2, 3, and 5), overlapping, with its anterior 

 superior angle, the posterior inferior part of the hyomandi- 

 bular, and leaving a considerable space between its upper 

 concave margin and the edge of the cranial shield. This is 

 the bone which has hitherto been called " operculum " in 

 Chondrosteus, and it certainly corresponds exactly in position 

 to the so-called operculum in Polyodon. It is, however, 

 equally clear that it corresponds also in position, as well as 

 in general shape, with the suboperculum of Paloeoniscus ; ^ 

 and that this is its true interpretation is proved by the dis- 

 covery of the proper operculum (op.) lying above it and 

 between it and the cranial shield (Figs. 2, 3, and 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 tranchiostegals (br.), which are 

 broad and plate-like where they immediately follow the 

 suboperculum, though anteriorly they become narrow and 

 slender. 



Jaws and palato-qvMdrate 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 3, it is curved inwards in front to meet its 

 fellow of the opposite side in a perfect symphysis; there 

 cannot, therefore, be a true prsemaxilla here any more than 

 in Aci'penser or Polyodon. Articulated to the oblique posterior 

 margin of the maxilla is a small flat plate (/), 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 



1 Described as " interoperculum " in my Memoir on the Structure of tlie 

 Palseoniscidffi. I have, however, abandoned that view, and now consider the 

 plate intercalated between it and the operculum in such genera as Bhabdolcpis 

 to be not a suboperculum, but merely an accessory element. 



