Royal Society. 423 



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 Clielonia. 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 

 these, the 9th, 10th, 11th, and the ' nuchal' plate are ossified from 

 independent centres, and remain permanently free from anchylosis 

 with the subjacent spines of the vertebrae : they are, therefore, 

 " dermal bones," homologous with those that overlie the vertebras 

 of the crocodile. But the first to the eighth neural plates inclusive 

 are serial homologues with the foregoing, and must, therefore, have 

 the same general homology. The objection that ossification extends 

 into their dermal cartilaginous basis from the neural spines is met 

 by the remark, that other parts, e.g. the radius and ulna of the frog, 

 are ossified from a common centre, without their homological di- 

 stinctness being thereby masked or destroyed. The course or start- 

 ing-point of ossification does not determine the nature and homology 

 of parts, and the author refers what he believes to be an erroneous 

 conclusion of Prof. Rathke to undue value being given to the cha- 

 racter of connation. 



The cartilaginous basis of the costal plates is developed in the 

 substance of the derm; the subjacent ribs are previously ossified and 

 present the normal slender form. But ossification extends from near 

 the head of each of the eight pairs of dorsal ribs, from the second 

 to the ninth pair inclusive, into the superincumbent dermal carti- 

 lages. This had been described as the development of the tubercle 

 of the rib. But Prof. Owen observes that, in the development of 

 the carapace of the young of the Testudo indica^ the connation of 

 the costal plate with the rib commences at a different point in each 

 rib alternately, and appears to be governed by the arrangement of the 

 horny scutes above. Another objection to these ossific expansions 

 being the tubercles of the ribs is presented by their abutment me- 

 sially against the neural plates, not against the vertebral diapo- 

 physes, as in the bird and crocodile. 



In regard to the development of the plastron, the author describes 

 two situations in which the primitive cartilages are developed, cor- 

 responding with those in the embryo-carapace, viz. one belonging to 

 the endo-skeleton, the other in the derm. The first form under 

 which the endo-skeletal parts of the plastron appear agrees with the 

 evidence aff'orded by the comparison of the fully-developed parts 

 with those of the crocodile, and proves the hyosternals, hyposternals 

 and xiphisternals to be 'hgemapophyses' or abdominal ribs: the 



