No: 1-] RELATIONSHIPS OF PROTOSTEGA. 33 
broad. In Protosphargis and Dermochelys the rib head is not 
covered by an expansion of the upper surface of the rib. 
The loss of the neurals may find its initial step in the condi- 
tion of Eosphargis (Chelone gigas Owen). There are in that 
genus, as described by Lydekker (30), six or seven large plates 
overlying the ribs; these were considered by Owen in the 
original description as neurals, but are considered by Lydekker 
as dermal scutes. It is difficult to see, however, whence the 
“median dorsal row of large carinated scutes”’ may have taken 
their origin if they are not neural plates which have lost con- 
nection with the vertebrae, and become laterally expanded so 
as to cover the ribs in part. This loss of connection between 
the neurals and vertebrae may be observed in the recent 
Lepidochelys kempt Garm., where 1, 2, 3, and also 8 are 
freer (21). 
The strong peripherals of Protostega were possibly present 
in Losphargis (30), and peripherals have been observed in 
Protosphargis (31). They were very slender in Protosphargis, 
and were considered by the original describer as phalanges, but 
were later shown by Baur to be peripherals. 
Most of the Chelontzdae have the typical number, eleven, but 
Thalassochelys and Lepidochelys have more, the number being 
varied by the introduction of one or two extra plates between 
the 5 and. 3 (22). 
The nuchal plate of Protostega differs widely from that of 
the living sea turtle, but in no point more widely than in the 
complete absence of the process on the under side for articula- 
tion with the last cervical. In Osteopygis, a sea turtle from the 
cretaceous, there is no trace of this process, but in Lytoloma, a 
form from the upper cretaceous and eocene, the eocene forms 
show the beginning of the process in a small tubercle (22). 
The carapace of Pvrotostega is now seen to be intermediate 
between the Dermochelytidae and the Cheloniidae, with several 
primitive characters which are ancestral to both. 
The vertebrae are represented by only two, from the caudal 
region. These are deeply concave in front, with the arch ossi- 
fied with the centrum. The anterior zygapophyses extend well 
forward of the anterior edge of the centrum, and the top of the 
