the, Head in Ormthosaurs. 23 



space more than §■ inch wide ; they extend some distance 

 forward into the bone ,• one is excavated for f inch ; each 

 measures ^ inch from side to side. Seen from behind, their 

 outlines are triangular ; thej are lined with smooth dense bone- 

 tissue. 



Such is this remarkable fossil. A fi-agment of a second 

 specimen has been placed in my hands by the com'tesy of 

 M. R. Prior, Esq., of Trinity College; but as it displays no 

 new structm'e, I merely mention that it indicates a bone twice 

 as large as that just described, and is perforated on the under- 

 side by foramina which are enormously large in proportion, 

 and which are accompanied by many small accessory fora- 

 mina. On the underside of neither specimen is there any 

 indication of division into separate frontal elements, though 

 externally both show indications of median lateral division. 



Now as to the significance of the bone. Its outline recalls the 

 frontal bone of Crocodiles (PL II. fig. 4). I figure for com- 

 parison the frontal bone of a Crocodile from the upper part of 

 the Tertiary series in the Isle of Wight, Crocodilus Hastingice. 

 Externally the Greensand fossil differs in the deep median 

 groove, in the smooth unpitted surface, and chiefly in the la- 

 teral parts being directed downward, while in Crocodiles the 

 lateral parts are directed upward. In the Oniithosaur the 

 bone is proportionally longer ; and the cerebral part being 

 broken, the resemblance is not so close as it seems to be. 



On the interior aspect the concave lateral parts of the Or- 

 nithocheirus are seen to be represented by similar but deeper 

 concave regions in the crocodile (PI. 11. fig. 4) ; for they are 

 the upper and inner boundaries of the orbits. Between them 

 is a similar triangular concave area, less well-defined ante- 

 riorly in the crocodile because the orbital margins do not 

 converge and meet in front. But here the resemblance ends ; 

 for when the bones are compared posteriorly, the crocodile 

 shows no sign of the remarkable excavations seen in the 

 Omithosaur*. 



Among birds the form of the bone is approximated to more 

 closely (PI. II. fig. 5). There is externally the same smooth 

 surface, the same sort of downward direction of the hinder 

 lateral parts, sometimes the same antero-posterior straightness 

 and mesial depression. These latter characters are not well 

 seen in the common Gallus domesticus, and might be better 

 matched in other birds ; yet, as the most accessible tyjje, I 

 here contrast (PI. II. fig. 5) the inner side of the frontal bones 

 in that animal with what is seen in our fossil^ premising that, 



* In serpents the frontal covers in the cerebral hemispheres in front. 



