No. 3.] ON INTERCALATION OF VERTEBRAE. 333 



observed similar cases as Albrecht's in Tfopidonotus. R. Owen^ 

 has also described a case of the same nature in the skeleton of 

 a Pytlioji tigris, of which he says: "Anchylosis has occurred 

 between the 148th and 149th vertebrae. The i66th and the 

 167th vertebrte have been more completely and abnormally 

 blended together, so as to seem but one vertebra on the left 

 side, where that half of the neural arch and spine have com- 

 pletely coalesced, whilst on the right side each vertebra supports 

 its own rib. A similar abnormality occurs between the 184th 

 and 185th vertebrae." 



If intercalation takes place at all, we ought to expect traces 

 of it in such forms as show a great increase in the number of 

 vertebrae, for instance, snakes, different groups of lizards, and 

 Plesiosaurs. In looking over such material I have found some 

 additional cases. 



In a specimen of Pelamis bicolor (No. 763, Yale University 

 Museum) I find the 212th vertebra simple on the left side, 

 double on the right side ; it bears one rib on the left, two ribs 

 on the right side. Exactly the same condition I have observed 

 in a cervical of a Plesiosaur, Cimoliasaurns plicatus. No. 48,001 of 

 the British Museum, London. One side has one, the other two 

 ribs. Mr. R. Lydekker ^ makes the following remark about this 

 vertebra : — 



" The centrum of a small and malformed cervical vertebra, 

 from the Oxford Clay near Oxford. This specimen is imma- 

 ture, and on one side is divided into two portions, each with its 

 distinct costal facet." 



1 do not doubt at all that in all these cases we have examples 

 of incomplete intercalation ; and I am convinced that this 

 intercalation is the result of the division of myotomes, as ex- 

 pressed by Albrecht. The possibility of such intercalation I 

 have expressed already in No. 306 of the " Zoologischer An- 

 zeiger," 1889, where I said : "Ich glaube, dass eine Verschmel- 

 zung oder Spaltung von Myomeren auch schon wahrend der 

 Anlage des Embryo moglich ist. Mein Freund A. Bohm in 

 Miinchen theilte mir mit, dass er verschiedene Anzeichen von 

 Spaltung von Myomeren beobachtet habe." 



^ Descriptive Catalogue of the Osteological Series contained in the Museum of the 

 Royal College of Surgeons of England, Vol. I, p. 123, London, 1853. 



2 Lydekker, Richard : Catalogue of the Fossil Reptiliaand Amphibia in the British 

 Museum, Part II, London, 1889, p. 238. 



