AST. 3 



COiSrCEENING THE ARMOR OF TURTLES HAY 



Thecal 

 ments 



ele- 



Thecophora 



Nuchal. 

 Neural plates. 

 Costal plates. 

 Marginal bones 

 Suprapygals. 

 Pygal. 

 .Plastrals. 



Thecal 

 ments 



ele- 



Epithecal ele-f Vf *^g'fl ^""^^'f^ /" 



ments *^^ .^^^^' ^^ ^ ^^^ 



I species. 



Epithecal ele- 

 ments 



Athecae 



Nuchal. 



Vestigial shreds on 

 costal plates. 



Plastrals, greatly re- 

 duced. 



[Upper and lower der- 



I mal armor, includ- 



I ing the ossicles of 



the marginal keels. 



When we consider the fact that the thecal elements of the Athecae 

 are nearly as much reduced as the epithecal of the Thecophora it 

 must be admitted, I believe, that the two groups are pretty widely 

 separated. 



3. ORIGIN OF THE PERIPHERAL BONES 



In my paper of 1922, on page 426, I suggested that the thecal 

 peripherals of the Thecophora may have arisen from a series of bones 

 at the outer ends of gastralia. At present I am inclined to look 

 on them as a row of bones developed one at the distal end of each 

 of the costal plates. The costal plates and these hypothetical pe- 

 ripherals would have the relative positions of the large dorsal and the 

 small lateral plates seen in the figures of Aetosaurus. The third 

 peripheral may be regarded as belonging to the first costal plate, 

 that overlying the second rib. The first rib is greatly reduced and 

 no costal plate is developed in connection with it. Nevertheless, 

 its distal plate may have bene retained as the second peripheral. 

 Usually no neural plate is developed which corresponds to the first 

 dorsal vertebra, but in some species of Trionychidae, as Aspideretes 

 gangeticus (Cuvier), there is present a plate of bone, the praeneural 

 which seems to belong with that vertebra. Ai present it appears 

 to me that the nuchal bone may be a plate homologous with the 

 neural plates and to have been in relation with the neural spine of 

 the last less cervical. In some ancestor a cervical rib may have been 

 overlain by a plate of bone, long ago absorbed ; but an accessory plate 

 at its distal end may have been preserved and have become the first 

 thecal peripheral. 



4. RELATION OF THE NUCHAL TO THE EIGHTH CERVICAL VERTEBRA 



Much importance is attached to the connection between the nuchal 

 plate and the neural spine of the eighth cervical in the leatherback 

 and the other sea turtles. If the writer's suggestion is correct that 

 the nuchal bone is a homologue of the dorsal neural plates the con- 



