Remains of the Pycnodont Fish, Mesturus. 3 



supposed basisphenoid, right postfrontal, right and 

 left squamosal, portion of right frontal, right and left 

 operculum, and other fragments. (PI. III. figs. 3-6.) 



I. Description of Specimens. 



The bones of the cranial roof form a continuous shield, 

 much arched from side to side, and extend from the occiput 

 to the rostrum without any fontanelle or loss of the tubercular 

 ornamentation. The sutures between them can be readily- 

 recognized on close inspection, and the limits of some of the 

 elements are remarkably inconstant. Tt is clear, indeed, that 

 these are merely dermal bones, and there is so much difference 

 between the cranial roof of the two best-preserved specimens, 

 that it is necessary to describe them separately. 



Specimen no. 1 *. — The hinder half of the skull in this fossil 

 (PI. I.) appears to exhibit its natural form ; but from the 

 middle of the orbits forwards it is much fractured and laterally 

 compressed. The occipital crest is not much elevated, and 

 the longitudinal median ridge of the roof is gently rather than 

 sharply bent from side to side. A few fixed points can be 

 definitely recognized, and it is necessary in the description to 

 begin with these for the approximate determination of the 

 more problematical bones. The occipital border of the cranium 

 is exposed behind by the crushing of the numerous small 

 supratemporal scales (PI. I. figs. 1,1a, s.t.) } and appears to 

 have been directly transverse, with the exception of a short 

 median peak formed by the supraoccipital element (s.occ). 

 The outer margin of the cranial roof is also well preserved on 

 each side, and the exact limits both of the otic region and the 

 orbits (orb.) are indicated. The margin of the supraoccipital 

 plate [s.occ.) is distinct except in the hindermost part of its 

 lateral sutures ; it projects posteriorly to fit in a triangular 

 notch of the median supratemporal, and is itself excavated in 

 front for the reception of the pointed , hinder extremity of 

 another median plate (p.eth.), which partly separates the 

 frontals. The latter elements are also readily recognized (,/K), 

 and meet in the middle line anteriorly for nearly one quarter 

 of their extreme length. Each frontal in this specimen is 

 much crushed and fractured ; but the sutures can be observed, 

 and a detached fragment from another specimen (Brit. Mus. 

 no. P. 6834) confirms the curious bifurcation of the hinder 



* A diagram of the arrangement of the cranial roof-bones in this fossil 

 is given in the recently published vol. iii. of the Catal. Foss. Fishes B. M, 

 p. 192, fig. 30. 



1* 



