G. R. Wieland—On Marine Turtles. 213 



supply these and other important details in the osteology of 

 these great turtles. Their discussion may, therefore, profitably 

 await the complete restoration of the types which I later intend 

 to give. 



The pleuralia are greatly reduced in size, being of even less 

 area than the neuralia, and fully as thin. They unite with 

 each other and with the neuralia by loosely interlocking digi- 

 tations. The pleural of the second rib in particular is thin 

 and small, occupying only about an eighth of the entire rib 

 length. (See figure.) 



The ribs are very characteristic, the first being remarkable. 

 The only specimen of this rib recovered is massive, and T4 cm 

 in length, or fully three-fourths the length of the second rib, 

 which is one meter long (see measurements). In the Chelo- 

 ninge (9) the length of the first rib, which is slender and reduced, 

 is only about one-fourth that of the second. The first rib in 

 Archelon is thus much more rib-like in general appearance than 

 in all other turtles with which it may be directly compared, 

 and must have extended out far enough for its distal end to 

 rest in the marginal groove, as in the case of the ends of the 

 other ribs. It is also quite possible that this rib was set even 

 more obliquely to the vertebral column than is shown in the 

 drawing. This point can later be determined. There is, how- 

 ever, no contact between this first rib and the first pleural, 

 Archelon differing in this respect from all other turtles except 

 Dermochelys. 



The second rib is also more massive than in any of the Che- 

 loninse, this being in full accord with the reduced small and 

 thin first pleural which it bears. In the case of the remaining 

 pairs of ribs, which are free for about four-fifths of their length, 

 the only character that need be mentioned now is the distal 

 increase in vertical thickness. The second to the fifth pairs of 

 ribs, inclusive, which are only about 2'5 CD1 in thickness where 

 they emerge from beneath the pleurals, have near their distal 

 ends a thickness of from four to five centimeters.* 



Discussion. — In Archelon, as has been noted, the neuralia 

 are strongly reduced in thickness, and the pleuralia in both 

 thickness and area. In fact neuro-pleural reduction is greater 

 than in the case of any other turtle known to possess all the 



* In the figure the ribs are drawn in the nearly flattened position in which 

 they were found, the first being the only one preserving the original curva- 

 ture as in life. The width of the carapace hence appears accentuated. 

 Nevertheless the general form of body was very robust. Indeed there 

 appears to have been an almost Trionychian rotundity, with the nuchal lying 

 more or less athwart the ends of the first pair of ribs (not the second as in 

 the Tri onychia), and projecting over from three to four of the cervicals as in 

 Dermochelys and certain Pleurodirans, as Pelomedusa. 



