No. 1.] RELATIONSHIPS OF PROTOSTEGA. 31 
The series with Dermochelys at one end, and Chelonia at the 
other, is marked by a constant variation in the size of the plas- 
tral elements, the nature of union of the bones, and the presence 
and position of the marginal processes. 
Carapace. — The vzbs are present in fragmentary condition 
in both specimens. The head was well developed and separated 
from the costal plate, the proximal end of which was expanded 
and produced into slender digitations. Examination of a speci- 
men of Chelonia mydas shows that the distance from the point 
of union of the rib head with the costal plate to the vertebral 
articulation is greater than the distance from the same point to 
the neural edge of the costal plate. In Pvotostega, as these 
specimens show, the opposite is true even when as in this case 
the measurements are carried only to the broken ends of the 
digitations. This shows that there was proportionally less room 
between the proximal ends of the costal plates in Protostega 
than in the living sea turtles, and in all probability too little 
room to allow the presence of neurals. This supposition is 
further borne out by the digitated proximal ends of the ribs 
and the entire absence of anything that can be referred to 
neurals in the known specimens. The expansion of the ribs 
extends for about half their length (Pl. VI, Fig. 19). 
The nuchal plate is very peculiar in form, resembling most 
nearly the nuchal of the soft-shelled turtles. In the present 
specimen the plate lies directly on the hyoplastra, having been 
crushed down on them, and has preserved them in their cor- 
rect relative positions; it is thickened in the middle, becoming 
thin laterally, and expanded into broad wings. The distal ends 
of the wings are irregular in outline, and probably articulated 
weakly with the first peripheral. The upper surface shows two 
low rugose ridges which run from the center out into the wings, 
and there disappear. The anterior edge was concave, and 
beveled from above downward and backward. The posterior 
edge is continued into a long, slender process running back 
over the vertebral column. The postérior end of the process is 
broken off, but apparently only a small portion is missing. 
There is no process on the under side for articulation with the 
posterior cervicals (Pl. V, Fig. 10). 
