[parks] CENTROSARUS APERTUS 57 



A faithful drawing of the skull is given herewith (Plate I) ; 

 it would be superfluous to redescribe the cranial elements as 

 they seem to correspond accurately with those of the specimens 

 previously referred to. It is to be observed, however, that the present 

 specimen has both supra-orbital horn cores well preserved and that it 

 has both the processes hanging over the fontanelles of the frill. The 

 type of Monodonius flexus has only the left and that originally figured 

 by Lambe only the right. Lambe's later figure, however, shows both 

 processes. 



The alveolus of the maxillary is 280 mm. long and contains 29 

 teeth of the character described by Brown for M. flexus. The mouth 

 is very narrow, the width between the cutting edges of the teeth in 

 the rear being only 122 mm. and much less anteriorly. 



The nasal horn core is very narrow, being 155 mm. antero- 

 posteriorly and only 57 mm. laterally near the base; it is perhaps 

 somewhat flattened. The general direction of this horn core is for- 

 ward but the tip is turned slightl}^ back, probably by distortion. 



The epoccipitals occur on each sinuosity of the frill and increase 

 in size from the front backwards. On the border of the frill, between 

 the parietal and the squamosal, Lambe's figure shows a separate 

 ossification of considerable size while Brown's shows an emargination 

 with broken edges, suggesting the loss of this element. In our 

 specimen this bone is little more than an enlarged epoccipital which 

 probably increases in size with the growth of the animal. 



Thyrohyals (Plate IV) are known in certain dinosaurs but, as 

 far as I am aware, these have not hitherto been found in the 

 Ceratopsia. Both these elements were discovered in the present 

 specimen with their anterior ends touching the internal surface of 

 the maxillaries near their posterior extremities, and the posterior 

 ends almost meeting in the midline. The bones are long and slender, 

 the right measuring 168 mm. and the left (slightly broken anteriorly) 

 162 mm. in length. The anterior end is somewhat expanded, the 

 upper surface flattish and the posterior end inflected slightly outwards. 

 The under surface is more convex in its anterior portion and this 

 convexity rises into a distinct ridge near the outer side of the bone less 

 than half way down. This ridge crosses the bone diagonally and 

 terminates on the inner edge at the posterior extremity, thus giving 

 a twisted appearance to the bone when viewed from the ventral side. 

 The width at midlength is 16 to 18 mm. and the thickness (dorso- 

 ventral) about 12 mm. 



The mandible (Plates TI and III) is worthy of especial mention 

 in that it shows a separate and distinct coronoid. This bone, as a sep- 



