G. R. Wieland — On Marine Turtles. 117 



that as a consequence our restoration is thus some ten centi- 

 meters too long. 



Summarizing then : there are to be seen on the midline 

 apparently overlapping all the neurals and the proximal bor- 

 ders of the laminae of the nine pairs of normal ribs which 

 pass out to meet the marginals, a series of eleven thin supra- 

 neural elements simulating in size and outline a neural series. 



These supra-neurals form a distinct median keel and are of 

 distinctly quadrangular outline all the way back to the eighth 

 and ninth, which are short on the median line, but nearly as 

 broad as the others. The digitate character of the sutures 

 between the successive members, but more particularly of the 

 pleural overlap, has already been commented upon. All the 

 outlines are quite exactly shown in my original figure (refer- 

 ence 6, Plate VI). A dominant feature of the midline is a nar- 

 row median groove which extends from the second to the 

 seventh member inclusive and is most pronounced in the sec- 

 ond and fifth. In the mid-region of each neural enumerated the 

 groove is somewhat widened and deepened, sending out a 

 radially ranged series of nutrition furrows or striations which 

 form a dominant sculpturing of the mid-region of the cara- 

 pace. Some further horny or even ossified elements may have 

 occupied the mid-region of these shields. The aspect of the 

 neural keel is thus seen to be different from that of any other 

 turtle. 



The supra-neurals of Archelon, be it noted, vary distinctly 

 from those of Toxochelys in which the series is not contin- 

 uous and corresponds to a normal series of vertebral horn- 

 shields. 



In Arehelon, however, a leathery hide must have been pres- 

 ent, with a system of keels of the usual number, as denoted 

 by dermogene ossifications rather than hornshields ; these will 

 be treated more fully later on. An ossicle like the supra- 

 neurals of Toxochelys was found by Hay accompanying 

 Protostega advena, but its derivation was left in doubt. 



The pleural investment of the ribs occupies only the proxi- 

 mal fifth of their length. The free ends of the ribs are thus 

 the dominating feature of the carapace. They are very heavy, 

 in compensation for the light to almost paste-board thickness 

 of the carapacial shield. The first rib is small and more or less 

 curved and flattened. As in Protostega, it passed well to the 

 front beyond the expansion of the first pleural, and may have 

 supported either the thin posterior nuchal ala or possibly some 

 osteodermal element. 



It is here necessary to note that the type specimen remained 



packed, partly in the matrix, from 1898 until 1906. Owing 



primarily, however, to a luckless defect in my field notes, 



Am. Jour. Sci.— Fourth Series, Vol. XXVII, No. 158.— February, 1909. 

 9 



