The Fossil Turtles of the Bridget Basin— Hay. 331 
chelydia, the Cryptodira, and the Trionychoidea. No 
Pleurodira have yet been found. 
The Amphichelydia are known in the Bridger beds by 
only the genus Baena. This genus was founded by Dr. 
Leidy, who described two species, B. undata and B. arenosa. 
Cope added B. hebraica and B. ponder osa, but the latter is 
based on unsatisfactory materials, and doubtfully belongs 
to the genus. 
The new materials belonging to this genus collected in 
1903 include several skulls, shoulder and pelvic girdles, limb 
bones, and cervical vertebrae. These specimens confirm 
the validity of Lydekker's group Amphichelydia, which 
was based especially on the shell of the English Pleuro- 
sternon, but regarded as including Baena. Pleurosternon 
is closely related to the Jurassic Compsemys plicatuia. 
Baur thought that the latter species was the ancestor of 
Baena ; but from the same quarries in the Jurassic that fur- 
nished Compsemys, the writer has described Probaena, a 
genus still more closely related to Baena. Probaena is also 
related to Platychelys of the Jurassic of Europe. In North 
America, Baena has now been traced back to the Judith 
River and the Belly River beds, therefore to the middle of 
the Upper Cretaceous. A study of the skeleton of Baena 
makes it certain that the group Amphichelydia furnished 
the ancestors of both the Pleurodira and the Cryptodira. 
The skull is almost wholly Cryptodiran, while the shell and 
the neck are Pleurodiran in structure. 
Remains of the species of this genus are among the 
most common fossils in the Bridger beds ; and since the 
shell is usually thick and the bones coossified the specimens 
are more likely to be well preserved than those of any other 
turtles. They were, without doubt, active swimmers, and 
able too to travel about on land. Their heads were short 
and broad, giving the animal a rather forbidding aspect. 
They were almost certainly addicted to the capture of living 
prey. Their habits and their appearance must have been 
much like those of our snapping turtle. 
Coming to the Cryptodira, we consider first the genus 
Baptemys. Only one species, B. wyomingensis Leidy, has 
been described. Leidy's type did not furnish a complete 
