ART. 3 CONCERNING THE ARMOR OF TURTLES HAY 11 



scutes is a coincidence and these are not the cause of the ossification 

 or of the forms of those plates. In the genus Testvdo alternate 

 costal plates are proximally broad, so as to articulate with three 

 neurals; intervening ones are narrow. In the embryo figured the 

 costals were preparing to assume those alternating forms. It will 

 hardly be contended that the shapes finally taken by the costal plates, 

 interlocking as they do, are determined by the horny shields. 

 Furthermore, the proximal end of each embryonic plate is pretty 

 certainly at the point where it quits the rib and reaches out to meet 

 the neural plate. If the growth of the costal is determined by the 

 epidermal shield the point where the rib becomes free ought to be 

 just below the outer extremity of the vertebral scute. A study of 

 the shells of a few species of tortoises will show no such relation. 

 The vertebral shields may be very broad while the rib-heads are 

 short. 



It is the contention of the present writer that the horny shields 

 of tortoises had primarily no relation to the costal plates, but to 

 more superficial bones, the epithecals. As a result of the suppression 

 of the epithecals the horny shields were brought into contact with the 

 more deeply lodged thecal bones. In Ghelys the epithecals are repro- 

 duced in many individuals and from these the horny scutes spread out 

 over the thecal bones. The only effect the scutes appear to have 

 on the thecals is to impress on their surface the radiating and con- 

 centric lines of growth. The shields do not grow at their edges 

 merely, but a new layer of horn is laid down on its whole lower 

 face, and these layers may often be separated from one another. 



