140 SHOULDER-GIRDLE AND BREAST-BONE. 



thoracic," then the "prse-abdominal" (p. r. a.), and lastly the "post-abdominal" plates (p. t. a.). 

 The Lizards, generally, retain the three foremost of the thoracic plates, and the Crocodilia the 

 azygous piece only; but, as I have shown, the "prse-" and "post-abdominal" plates reappear in 

 the latter but broken up into a series of pseudo-ribs. The form of the interclavicular piece of the 

 Lizard is curiously copied in the Chelonian, but broken up, as it were, the transverse bands 

 appearing in Trionyx (Plate XII, fig. 13, i. cl.), and the longitudinal piece (as in the Crocodile) in 

 Chelone virgata (Plate XII, fig. 14, i. cl.) ; in Spharyis (figs. 11 and 12) the interclavicle is wanting. 



These plates take on an essentially ganoid character in certain Mud-Tortoises (see a figure 

 of the plastron of Tetrathyra Baikii, in Dr. Gray's paper " On a new Genus and Species of the 

 Trionychidse from Western Africa," 'Zool. Proc./ 1865, part i, pp. 323, 324). In them the 

 interclavicle is V-shaped, and answers to the arms of that of the Varanians (Plate X), whilst 

 the clavicles have a "ganoid" disc on their free ends, and the post-thoracic plates have a much 

 larger growth of the same kind. 



If we look at the skeleton of the Common Tortoise (Testudo grceca} we shall see that the 

 endo-skeleton is really very delicate, whilst the dermal ossifications are exceedingly strong, and 

 fit into each other by sharp sutural teeth. Now, if these dermal plates had been broken up into 

 innumerable small disks, as in the Blind-worm (Awguis fragilis}, and each of these defended 

 externally by its own epidermal scute, and if we also suppose the limb- skeleton to have been 

 absent, then the Tortoise would have been a mere Snake, broad-bodied, indeed, and horny faced, 

 but a Snake notwithstanding. The dermal skeleton of the Chelonian is to that of the Anguian 

 or Cyclodont what the ganoid covering of CallicJtthys or Gasterostcus is to the outer skeleton 

 of a Fish with smaller imbricated scales or plates, whether " ganoid," " cycloid," or " ctenoid." 



Now, if we compare the dermal armour of CallicMhys (see Plate I, figs. 9 13,) with that 

 of a Chelonian, leaving out of question, for the time, the borrowed origin of many of the dorsal 

 and supero-lateral plates of the latter, we cannot choose but see a great correspondence. In 

 CallicMhys (Plate I, fig. 10), we have the dorsal or spinal plates, then the supero-lateral plates 

 (fig. 9), autogenous in the Fish, but connate with the ribs in the Tortoise. 



The marginal (supplementary) plates of the Tortoise are not generally represented in Cattich- 

 thys, but this segment appears on the right side of the Shoulder-region (Plate I, fig. 9, s. cl.) ; it is 

 the " supra-clavicle." The azygous piece of the Chelonian Plastron does not appear in that form 

 in the Fish not in any Fish ; but in the Shoulder-region we have its true counterpart as a pair 

 of intercalary plates (Plate I, figs. 9, 11, 12 and 13, i. cl.). Certain Fishes have azygous plates 

 on the thoracic-abdominal line ; but these, as I have shown, belong to a further breaking up 

 of the dermal system, and are wedged in between the paired " inter-clavicles." 



In those ancient relatives of the Chelonia, the Plesiosaurs, the azygous bones along the 

 abdominal line are evidently the serial homologues of the inter-clavicle (the so-called " ento- 

 sternum") of the Chelonian. 



The Shoulder-girdle of the Chelonian is extremely simple ; that of the ripe embryo of 

 Green Turtle is shown in Plate XII, figs. ! 3 ; the figures are magnified two diameters. If 

 we seek for likenesses to the condition of the parts seen here, we must look backwards to the 

 Amphibia and forwards to one genus of the Bird-class, viz. Struthio. In Chelone mydas each 

 moiety is merely a forked ray, the bifurcation of which takes place at the middle, at which part 

 there is considerable enlargement, the swollen part being cupped deeply on the outside ; this is 

 the glenoid cavity (gl.) ; the main part is the scapula (sc.) ; the front fork the prse-coracoid 

 (p. cr.); and the hinder fork the coracoid (cr.). I have given but one instance (Dactylethra, 



