Observations on the Structure of Mesosaurus. 109 



ADDENDUM. 

 (September 1, 1904.) 



When writing the above paper I refrained from offering any 

 opinion on the question of the systematic position of Mesosaurus, in 

 the hope that a good skull might turn up and settle the question 

 beyond dispute. As, however, different opinions on the question 

 have been recently expressed by Osborn and Boulenger, and the 

 question has an important bearing on the classification of Eeptiles, 

 it may perhaps be advisable to consider the direction in which the 

 evidence derived from the pectoral and pelvic arches seems to point. 



By Osborn Mesosaurus is placed in his Super-order Diaptosauria 

 — a group which includes the primitive Ehynchocephalians of other 

 writers — and the Plesiosaurs he regards as having been derived 

 quite independently from Cotylosaurian ancestors. By Boulenger 

 Mesosaurus is believed to be closely related to the early Plesio- 

 saurians, such as Neusticosaurus, and the whole Plesiosaurian order 

 to be descended from Ehynchocephalian ancestors. 



While the whole question cannot be finally settled except by the 

 skull, the evidence of the limb girdles seems to lead us a certain 

 distance on safe ground. In Mesosaurus the pubis and ischium are 

 plate-like and very similar to those bones in Procolopkon and Palceo- 

 hatteria. The pectoral arch also seems to be a modification of the 

 Procolopkon type. We may therefore conclude that Mesosaurus is 

 descended from a land animal that had plate-like pubis and ischium 

 and an ossified precoracoid. Such a land animal might be found in 

 the Microsauria, Cotylosauria, or the Diaptosauria. That Mesosaurus 

 is directly descended from a Microsaurian ancestor is very impro- 

 bable for a number of reasons. That it is descended from a Cotylo- 

 saurian is not at all improbable, but it is much more likely that the 

 ancestor was a primitive Ehynchocephalian or Diaptosaurian. And 

 on this point Osborn and Boulenger seem to be agreed. But on the 

 question of the affinities of Mesosaurus to the Plesiosaurs their views 

 are very dissimilar. 



The Plesiosaurs are so extremely specialised in many directions 

 that the determination of their affinities has been a matter of great 

 difficulty, and many very different results have been arrived at. 

 Many writers have been impressed by the Chelonian resemblances 

 in some of the characters, and have regarded the affinities as mainly 

 Ehynchocephalian. The skull, however, bears much resemblance to 

 that of the Anomodonts, and hence the Plesiosaurs have been re- 

 garded as more closely allied to the Theriodonts and Anomodonts 



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