ig BAUR. [Vol. VII. 



Some Remarks on the Relations of the MosasauridcB. 



It is evident from the description of the elements of the skull 

 of Platecarpus given above, that the Mosasauridae must be con- 

 sidered as a family of the Lacertilia, without any relations with 

 the Ophidia. The question now is, what rank do the Mosasau- 

 ridae have among the Lacertilia ? By nearly all authors, the 

 Mosasauroids are considered a sub-order of the Squamata, like 

 the Dolichosauria, Chamaeleontia, Ophidia, for instance ; but it 

 seems to me that this is not correct. There cannot be any 

 doubt that the Mosasauroids are nearest to the Varanidae, — 

 nearer than to any other group of Lacertilia. The whole skel- 

 eton is Varanoid ; and I feel quite confident that Varanidae and 

 Mosasauridae developed from a common ancestor, which was 

 already a typical Lacertilian.^ This ancestral group must have 

 existed during the upper Jurassic or the lower Cretaceous time. 

 The Mosasauroids became true marine animals. The Varanoid 

 limbs were transformed into fin-like limbs similar to those of 

 the sea-tortoises, but still more adapted to the water. At the 

 same time they reached great size, like all higher vertebrates 

 which are transformed from terrestrial to sea-animals. 



In Science of Nov. 7, 1890, I have expressed the opinion 

 that the Mosasauridae are very closely related to the Varanidae. 

 I said : " They simply represent highly specialized aquatic forms. 

 The Helodermatidae belong to the same group, but the Mosa- 

 sauridae are very much nearer to the Varanidae. For this group 

 I retain the old name Platynota, and divide it into two super- 

 families, — {a) Varanoidea, i. Varanidce,2. Mosasaiiridcs ; [b) Helo- 

 dermatoidea, i. HelodermatidcE." Since this was written a paper 

 has appeared by Mr. Boulenger^ in the Proceedings of the 



1 I may state here that the restoration of the shoulder-girdle given by Professor 

 Marsh, and copied since that time in different handbooks, is not correct. The cora- 

 coids did not meet in the middle line as in the Plesiosauria, but were exactly as in 

 the Lacertilia. There were very large cartilaginous portions of the coracoids which 

 overlapped each other, and with these portions the sterum was connected. The 

 sterum was not ossified, but simply calcified, as in the Varanidae and other Lacer- 

 tilians. The scapula shows the original simple form which is also seen in the allied 

 Helodermatidse. 



2 Boulenger, G. A , N'oles on the Osteology of Heloderma horridum and H. sus- 

 pectum, with Remarks on the Systematic Position of the Helodermatidce and on the 

 Vertebra of the Lacertilia, Proc. Zool. Soc. Lond., 1891, pp. 109-I18. 



