190 CRITIQUES AND ADDRESSES. [ix. 



in presenting true articular surfaces for the free jointing 

 of the bones of the fore-arm. In the apparently com- 

 plete absence of hinder limbs, and in the characters of 

 the vertebral column, the Zeuglodon lies on the cetacean 

 side of the boundary line ; so that, upon the whole, the 

 Zeuglodonts, transitional as they are, are conveniently 

 retained in the cetacean order. And the publication, in 

 1864, of M. Van Beneden's memoir on the Miocene and 

 Pliocene Squalodon, furnished much better means than 

 anatomists previously possessed of fitting in another 

 link of the chain which connects the existing Cetacea 

 with Zeuglodon. The teeth are much more numerous, 

 although the molars exhibit the zeuglodont double fang ; 

 the nasal bones are very short, and the upper surface of 

 the rostrum presents the groove, filled up during life by 

 the prolongation of the ethmoidal cartilage, which is so 

 characteristic of the majority of the Cetacea. 



It appears to me that, just as among the existing 

 Carnivora, the walruses and the eared seals are inter- 

 calary forms between the fissipede Carnivora and the 

 ordinary seals, so the Zeuglodonts are intercalary between 

 the Carnivora, as a whole, and the Cetacea. Whether 

 the Zeuglodonts are also linear types in their relation to 

 these two groups cannot be ascertained, until we have 

 more definite knowledge than we possess at present, 

 respecting the relations in time of the Carnivora and 

 Cetacea. 



Thus far we have been concerned with the intercalary 

 types which occupy the intervals between Families or 

 Orders of the same class ; but the investigations which 

 have been carried on by Professor Gegenbaur, Professor 

 Cope, and myself into the structure and relations of the 

 extinct reptilian forms of the Ornithoscelida (or Dino- 

 sauria and Compsognatha) have brought to light the 

 existence of intercalary forms between what have hitherto 



