Geological Society. 209 



GEOLOGICAL SOCIETY. 

 [Continued from p. 147.] 

 January 7, 1857. — Colonel Portlock, R.E., President, in the Chair. 

 The following communications were read : — 



1. " On the D/cAorfort cuspidatus." By Professor Owen, F.R.S., 

 F.G.S. 



In this paper additional facts were communicated relative to the 

 dentition of the Dichodon cuspidatus to those given in the author's 

 original memoir on the species in the ' Quarterly Journal of the Geo- 

 logical Society,' vol. iv. p. 36. They related to the structure of the 

 last molar tooth of the lower jaw, which has a third hicuspid lobe, 

 and to the forms and period of succession of the permanent teeth. 

 The formula of the deciduous dentition was 



i^^, c'-=l, dmt:^=32; 



3—3 1— l' 4—4 



m'-^ =44. 



3—3 



that of the permanent dentition is 



;2=?, cl^, p'-^, 



3—3 1 — l' ^ 4—1 



The form and structure of all the permanent teeth, with the excep- 

 tion of the fourth jDremolar, are now known. 



The deciduous formula is the same as in the genus Siis ; the per- 

 manent one differs by the displacement of the first deciduous molar 

 by a true premolar. Whilst, however, the Dichodon resembles the 

 Hog-tribe in the kind and number of its teeth, it resembles more 

 the Ruminantia in the configuration of the true molars. . The now 

 ascertained facts of the deciduous dentition of the Dichodon supply 

 an additional test of its affinities, owing to the marked difl^^erence in 

 the times and order of succession of the permanent teeth, between 

 the non-ruminant and the ruminant Artiodactyles. 



In the Ox and Sheep the last true molar cuts the gum before any 

 of the premolars appear, and the canine teeth are the last to come 

 into place. In tlie Hog the canines appear before the premolars, 

 and these rise into place before the last molar has attained the level 

 of the other grinders. In the Dichodon the second true molar is in 

 place before any of the deciduous teeth ha,ve been shed ; and it more 

 resembles the Sheep in the order of appearance of the permanent 

 teeth. But there is some difference ; for by the time the second 

 true molar is as far advanced in development as in the Dichodon, 

 figured in plate 4 of vol. iv. Quart. Journ. of Geol. Soc, the first 

 permanent incisor is in place, and the germs of the premolars in the 

 formative cavities have calcified crowns. Tlie adult Dichodon appears 

 to have equalled in size a South Down sheep : the probable age of the 

 immature individual above referred to might be surmised from the 

 analogies presented by the subjoined Table of the times of appear- 

 ance of the permanent teeth in the — 



