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 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 
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 vertebre: they are, therefore, 
“dermal bones,” homologous with those that overlie the vertebre 
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 oi 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 ossifie 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 afforded by the comparison of the fully-developed parts 
with those of the crocodile, and proves the hyosternals, hyposternals 
and xiphisternals to be ‘hamapophyses’ or abdominal ribs: the 
