792 



contact with that part of the aurora towards the east. When the 

 aurora commenced, the moon was considerably below the horizon ; 

 but this, it is considered, does not form any serious objection to 

 what has been stated, since the aurora soared to so great a height, 

 that the rays of light proceeding from her would strike the aurora a 

 considerable time before she arose above the horizon. 



The aurora continued for upwards of six hours, and during that 

 time the thermometer stood at 34"^. 



January 18, 1849. 

 The MARQUIS OF NORTHAMPTON, V.P., in the Chair. 



A paper was read, entitled " On the Development and Homolo- 

 gies of the Carapace and Plastron of the Chelonian Reptiles." By 

 Professor Owen, F.R.S. 



The author commences by defining the several parts of which the 

 osseous thoracic-abdominal case of the Chelonian Reptiles is com- 

 posed, and briefly discusses the several opinions that have been 

 published with regard to their nature and homologies, dwelling 

 chiefly on that recently proposed by Prof. Rathke, in his work on 

 the Development of the Chelonia, in which it is contended that the 

 carapace consists exclusively of the development of parts of the 

 endo-skeleton, viz. the neural spines and vertebral ribs (pleurapo- 

 physes)^ agreeably with the opinion of Cuvier and Bojanus, and 

 that the remainder of the thoracic-abdominal case, consisting of the 

 "marginal pieces" and "plastron," are formed entirely from bones 

 of the dermal system. 



Adverting to the hypotheses of Cuvier, Geoffroy and Meckel, that 

 the thoracic-abdominal case is a modification of parts of the endo- 

 skelelon exclusively, the author tests their determinations by com- 

 parisons with the corresponding parts of the bird and crocodile, and 

 infers, from the latter animal, that the hyosternal, hyposternal and 

 xiphisternal bones are not parts of the sternum, but are homologous 

 with the hasmapophyses (sternal ribs and abdominal ribs) ; those in the 

 Plesiosaurus making the nearest approach to the peculiar develop- 

 ment of the parts in the Chelonia, especially as they appear in the 

 plastron of the immature Terrapenes and Sea-turtles. 



Admitting that any hypothesis framed from the comparison of the 

 completed structures in the adult Vertebrata requires for confirma- 

 tion its agreement with the important phenomena of the develop- 

 ment of those structures, the author proceeds to apply that test. 



He details his observations on the development of the skeleton, 

 and especially of the thoracic-abdominal case, in the embryos and 

 young of different genera of Chelonia. The chief facts that have 

 governed his conclusions are the following : — 



With respect to the carapace. The cartilaginous basis of the 

 neural plates is developed in the substance of the derm ; and of 



