REPTILES. 133 



"CHELONIA" (Tortoises and Turtles). 





 i 



.Each moiety of the Shoulder-girdle a bifurcated rod, the * mnder fork only having a separate 

 ossification from the main bone ; endoskeletal part of the thorax open below ; clavicles and 

 interdavide forming the three anterior plates of a thoracic- abdominal (dermal) shield, which 

 is composed altogether of nine bones. 



Example. Chelone mydas, Linn. 



In the abstract of the present Memoir, ' Proc. Zool. Soc., 1864,' part 3, p. 339, I 

 controverted Professor Owen's view of the morphologically compound nature of the Plastron of 

 the Chelonia. 1 Professor Owen expressly says ('Ost. Catal. Hunt. Mus.,' vol. i, p. 169), that 

 ' The parial pieces of the Plastron are the ' b.8emapophyses 5 connate with expanded dermal 

 ossifications." In my paper (p. 340) I said, " There is no connation whatever ; there is no 

 Sternum at any time, and no hgemapophyses ; nothing, indeed, but membrane-bones formed 

 between the corium and the membrane lining the abdominal cavity. One bone, the azygous 

 piece, answers to a similarly un symmetrical piece in the thoracic apparatus of the Aves and of the 

 Lacertilia, and had its counterpart also in the extinct Plesiosaurs and Ichthyosaurs, and also 

 exists in the Monotremes." So far true : then I went on to mistake the furcula of the Bird 

 the fibrous part of it for a single piece, and to confound the clavicles of the Lizard with the 

 prse-coracoids of the Frog. Nor was I then aware that the so-called "episternals" of the Chelonian 

 are really the "clavicles." 



But the reduction of the Plastron to its true dermal simplicity was a right step ; it was 

 generously appreciated by Professor Huxley at the time, who at once put Rathke's splendid work 

 on the Anatomy of the Tortoises into my hand. This work (' Ueber die Entwickelung der 

 Schildkroten/ 1848) appeared one year before Professor Owen's paper was published in the 

 ' Philosophical Transactions.' I now was made aware 2 that my observations had been forestalled 

 some sixteen years, through a much more extended study of these creatures by the great German 

 embryologist. If, however, Professor Owen had not supposed a " connation" of endo- and exo- 

 skeletal parts in the Plastron his paper would have been almost faultless ; and, in spite of this 

 hypothetical deduction, it is of very great value. With regard to the connation of a large number 

 of the ribs and neural spines with the supero-lateral and upper plates, of that there can be no 

 doubt. In plates iii, v, and vi of Rathke's work, the continuity of the rib and the great dermal 

 plate overlying it is shown again and again. My own observations agree with those of both 

 authors (see Plate XII, fig. 6), which shows a transverse section of a rib of the eleventh vertebra 

 of the ripe embryo of a Green Turtle (Chelone mydas) magnified ten diameters. A large cartilaginous 

 core is seen within the "ectosteal" sheath ; this sheath has grown more outwards than inwards, and 

 shows a perfect circle of large diploe cavities, and then it is seen to spread both fore and aft into the 

 looser inner part of the thick corium. In the same Plate (fig. 5), the terminal free part of the 



See Owen "On the Development and Homologies of the Carapace and Plastron of the Chelonian 

 Reptiles," ' Phil. Trans./ part i, 1849 ; and < Ost. Catal. Hunt. Mus./ vol. i, pp. 168 170, No. 769. 



The part relating to the structure and development of the Plastron was kindly translated for me 

 by Mr. Power. 



