46 THE ROYAL SOCIETY OF CANADA 



depressions and grooves. The lower surface is evenly concave from 

 side to side and relatively smooth, and shows no trace of coossifii cation 

 along the midline. The plate throughout is moderately thin, attaining 

 its maximum thickness alongside the groove at midlength, and its next 

 greatest thickness at the middle of the lateral keels. It thickens along 

 the side and front borders, but to a greater extent laterally than in 

 front. The hinder border has an undulatory outline, comes to a com- 

 paratively thin edge, and is rugose on the under surface for a short 

 distance forward in a marginal area defined in front by a broad, 

 shallow, but clearly marked transverse sulcus reaching across the 

 plate. The edge of the plate laterally and in front is channelled by a 

 peripheral groove which is most strongly developed where the border 

 is thickest. 



Measurements of Nuchal Plate of Type. 



mm. 



Maximum distance transversely between lateral borders. 272 



Maximum breadth following curves of upper surface 330 



Length at central groove marking line of coossification 203 



Thickness at midlength adjoining central groove 31 



Thickness at midlength of lateral ridge 27 



Thickness along anterior border , 15 



Thickness of lateral border in advance of maximum breadth .... 23 



This composite plate was found close behind the skull, slightly 

 out of alignment but practically in place, with its front border about 

 two inches from the hinder border of the occipital plate of the head. 

 The front border of the nuchal plate and the back border of the occip- 

 ital one conform to each other in general curvature. When in place 

 the nuchal plate is nearly on a level with the top of the skull. It has 

 almost the same breadth as the skull, and the transverse convexity 

 of the upper surface in each is similar. The proximity of this large 

 plate to the head, and the fact that the atlas, axis, and third cervical 

 vertebra, covered by the plate, are coossified indicate that there could 

 have been little lateral movement of the head. 



The plate that is thought to have followed behind the nuchal 

 plate resembles the latter in general appearance, and need not be com- 

 mented on further than to say that it has practically the same length 

 but is a little broader with less transverse curvature, and shows some 

 slight differences of outline. Its transverse curvature has the appear- 

 ance of being normal. In this plate also, the union of its two halves 

 has left no trace on the under surface beyond a slight thickening of 

 the bone beneath the groove of the upper surface. 



