793 



these, the 9th, 10th, 11th, and the ' nuchal' pla^e are ossified from 

 independent centres, and remain permanently free from anchylosis 

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

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

 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.^. 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 difi'erent 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 ' hsemapophyses' or abdominal ribs: the 

 hyosternals and hyposternals are primitively long, slender, trans- 

 verse bars, which join the vertebral ribs in the Tortoises and Terra- 

 penes, without the intervention of any marginal pieces. The ossifi- 

 cation of the superadded dermal portions proceeds from the pre- 

 viously ossified endo-skeletal elements. 



The author concurs with M. Rathke in regarding the marginal 

 pieces as ' dermal bones,' and concludes by a full discussion of the 

 facts and arguments which have led him to a different conclusion 

 respecting the nature and homologies of the carapace and plastron. 



The memoir is illustrated by figures of the carapace and pla- 

 stron, and of the corresponding segments of the skeleton in the bird 

 and crocodile, and of the development of the thoracic-abdominal case 

 in land- and sea-chelonians. 



