Remains of the Pycnodont Fish, Mesturus. 9 



half as large as the others ; and as these pass backwards they 

 become more and more irregular, are supplemented with more 

 intermediate teeth, the majority still smaller and all very 

 irregularly arranged. The dentition on the splenial bone 

 (PL I. figs. 1, 1 c; PL II. fig. 3) extends to its outer margin 

 as far as the base of the coronoid process. There is one 

 principal longitudinal series of oval teeth, about as large as 

 the median vomerine series, but the long axis of each tooth 

 oblique. The outermost row of teeth (wanting in the original 

 of PL I.) is not much inferior in size, and, as in the vomer, the 

 external end of each tooth is produced into a slight peak for 

 grasping. Within the principal series one regular row of 

 round teeth about half as large and an innermost irregular row 

 of still smaller teeth can be observed. Between the principal 

 and outermost series the same small teeth, disposed almost 

 symmetrically with those on the inner side of the principal 

 series, are also seen ; but, except in the anterior third of the 

 bone, these two rows are supplemented and partl)^ disturbed 

 by the interposition of other small teeth. All these tritoral 

 teeth in both jaws are smooth and exhibit a shallow apical 

 indentation, with a crimped margin, very rarely also with a 

 slight median tubercle. The shape of the indentation varies 

 with that of the tooth, and the hinder portion of its crimped 

 border is not infrequently wanting. 



Opercular and Branchiostegal Apparatus. 



The opercular apparatus is well displayed, and the oper- 

 culum and preoperculum can be readily identified. On the 

 left side of no. 2 these bones are crushed a little downwards 

 and the upper end of the hyomandibular is thus partly ex- 

 posed, showing the short and deep process for the suspension 

 of the operculum. The last-mentioned plate is also preserved 

 among the isolated bones in Brit. Mus. no. P. 68/^4 (PL III. 

 figs. 5, 6). It is comparatively small, deep and narrow, its 

 truncated upper end twice as broad as the suprateraporals 

 (shown in PL I. fig. 1), its straight anterior border placed 

 vertically at the line of the occiput, and its lower half tapering 

 to a point. In front it is overlapped, as usual, by the pre- 

 operculum {p.op.), and there is a large hollowed articular 

 facette (PL III. fig. 5 a,f.), well within on its anterior border 

 near the upper extremity, for its suspension on tlie hyoman- 

 dibular process. The preoperculum is the large irregularly 

 triangular plate always conspicuous in Pycnodont skulls, 

 and in no. 2 its hinder margin distinctly corresponds with 

 that of the hyomandibular. On the right side of no. 1 



