ART. 3 CONCERNING THE ARMOR OF TURTLES HAY 7 



6. RELATION OF THE COSTAL PLATES TO THE RIBS 



In the carapace of the thecophorous chelonians the broad costal plates 

 are intimately fused with the underlying ribs. If my explanation of 

 the construction of the carapace is correct, those costal plates at some 

 time in the history of these animals were free from the ribs; also it 

 is probable, or at least possible, that in the embryologic development 

 of some existing species the costal plates will be found to arise by 

 distinct centers of ossification and only later to fuse with the ribs. 

 Eminent naturalists have argued on this side of the question ; others 

 on that; a few, possibly, on both sides. Apparently Goette was the 

 first to make a thorough investigation of the embrj^onic development, 

 and he appeared to prove that the costal plate had in it no element 

 of dermal bone. Nevertheless, Volker found himself driven to con- 

 clude that Goette was in error. A Japanese naturalist, Ogushi,* 

 working on a species of soft-shelled tortoise {Trionyx)^ found that 

 Goette's explanation compelled the conclusion that the scapula, which 

 in other vertebrates overlies the ribs, has been brought to articulate 

 by its distal end with the underside of the second rib. For this and 

 other reasons Ogushi rejected Goette's hypothesis. Joan B. Proctor,^ 

 in studying the early stages of the remarkable land tortoise, Testudo 

 lovendgii, found important evidence that the costal plates originated 

 independently of the ribs. 



7. RELATION OF THE HORNY SCUTES TO THE UNDERLYING BONES 



Volker (his page 523) discusses the relations of the horny scutes 

 to the underlying bones. He agrees with me that primitively the 

 scutes coincide with the epithecal ossicles and that now in the the- 

 cophorous turtles the coincidence no longer exists. Each horny 

 scute may cover parts of from two to as many as 10 bones. In my 

 paper of 1898 I connected this expansion of the scutes with that of 

 the epithecal bones, expressing the view that these bones may once 

 have occupied most of the space now covered by the horny scutes 

 of the living turtles. It is, however, not necessary to suppose that 

 they were so large; although, to judge from Chelys, some of them 

 must have had a respectable size. It can hardly be doubted that the 

 scutes of the Pleurodira and the Cryptodira had their origin on the 

 dominant epithecal bones of the keels of their early ancestors. In 

 the primitive Thecophora the bones of the deeper layer were gaining 

 the ascendency at the expense of the superficial ones. Although the 

 expansion of the epithecal ossicle was checked, the overlying scute 

 continued to grow. We must suppose further that the space between 

 the keels was in some cases occupied by small plates of bone, as now 



*Morphol. Jahi-b., vol. 43, 1911, pp. 13-15. 



« Proc. Zool. Soc. Lond., 1922, pp. 483-526, pis. 1-3, 21 text-figs. 



