No. 1.] RELATIONSHIPS OF PROTOSTEGA. 43 
ahnliche Ausbildung ergeben wiirde.” It is placed among the 
Protostegidae wpon the evidence of the descending processes; 
but, as will be shown later, the flat, wide skull has a strong 
resemblance to some of the Dermochelyidae. 
No forms of this family have shown the presence of derma! 
ossifications in the carapace, but all, in which the carapace is 
known, do have peripherals which are unknown by observation 
in the Dermochelyidae. 
The nuchal plate of the middle cretaceous form, Osteopygis, 
is known to lack the process on the under side for the last 
cervical vertebra. The other forms, probably, were devoid of 
the process. 
The Dermochelyidae have a carapace formed of dermal ossifi- 
cations, no peripherals, and an entire lack of the descending 
plates of the parietals. The earliest known form, Zosphargis, 
from the eocene, has the carapace represented by a median row 
of scutes which are, possibly, the loosened and expanded neural 
plates; the peripherals were supposed to have been present by 
Lydekker (30). The skull shows no trace of the parietal plates, 
and is broad and flat. 
The next form, Psephophorus, ranges from the eocene into 
the miocene. It has the tesselated dermal ossifications of 
the carapace already well developed, no peripherals, and the 
humerus very similar to that of Dermochelys. 
The Cheloniidae afford better material for comparison. The 
earliest form, Osteopygis, from the middle cretaceous, had eleven 
peripherals, 2, and 11 were free from rib attachment, 2 and 
8 had deep pits for attachment to the plastron. Between these 
there was asmall fontanelle. The carapace was practically closed. 
Allopleuron (Chelone Hoffmant Gray) of the upper cretaceous 
presents an evident offshoot from the true line of the Chelonizdae. 
The carapace was long and narrow, the nuchal deeply emargi- 
nate, and the neurals short and wide with a long keel. The 
pleurals are of considerable antero-posterior extent, but are con- 
fined to the proximal ends of the ribs, which are very slender. 
The nuchal shows no process on its under side. The periphe- 
rals are long and slender. The posterior nares are located far 
back. 
