400 G. R. Wieland — Archelon ischyros. 



imperfectly by means of loosely doubled interlocking and over- 

 lapping digitations grading into frequent free spines posteriorly. 

 These digitations are mostly long, thin and ribbon-like, and 

 produce a junction quite different from the usual suture. In 

 many cases there is an appearance such as would result if the 

 digitations of the one plate had lain upon the surface of the 

 adjoining plate when it was in a plastic condition and thus 

 raised rounded ridging about their edges. The order of the 

 digitations and their size is rather irregular. 



With the exception of the medial pair of plates correspond- 

 ing to the eighth dorsal vertebra and the single medial follow- 

 ing, which are small and crowded, and the pygal, which is 

 comparatively large and heavy, the plates of the carapace are 

 quite regular in size. The width and thickness are quite the 

 same throughout, with a shade more of development of both in 

 the pelvic region. One of the chief features of the carapace is 

 the arching into a heavy dorsal ridge, beginning just back of 

 the region of the first dorsal vertebra, and from thence con- 

 tinuous, except in the sacral region. It marks the position of 

 the neural spines, and is very distinctly grooved from anteri- 

 orly to the region of the eighth dorsal vertebra. Immediately 

 over the neural spines this groove inclines to widen, deepen 

 and send out asteriations. In life, these grooves were no 

 doubt filled with horny material, and the animal may have 

 borne a dorsal row of spines. It is the appearance along this 

 groove that makes me regard the medial carapace as consisting 

 of a double row of plates. If these are not to be regarded as 

 separate ossifications, the whole number of plates wonld be 

 28, consisting in the nine pairs which are morphologi- 

 -cally"^ simply uncinoid expansions of the ribs, and a medial 

 row of plates, ten in number. I shall have more to say about 

 the carapace at a subsequent time. It consists of very thin, 

 bnt dense and splintery, bone imbedded in exceedingly hard, 

 indurated marl. In one section, through the medial dorsal 

 ridge just back of the third rib, there is a large open space 

 above the centrum, the thickness of the carapace being in no 

 place much over •5'^'^. At another break, exposing in section 

 more nearly through the end of a centrum, back of the sixth 

 rib, the outer carapace may be said to form a very open Y, 

 consisting of very thin layerings, while there is a very thin 

 laminated to honeycombed connection with the summit of the 

 underlying neural spine •3'^°' in thickness by V"^ in vertical 

 height. There is a tendency to very small thin and irregular 

 bony projection from both bases of this connection. In the 



* On relation between carapace and endoskeleton, see Baur, Science, March 23, 

 1888 ; also a much more extended treatment in the Biologische Centralblatt, Nr. 5, 

 ■6, 1889 (Erlangen). 



