The Fossil Turtles of the Bridget Basin— Hay. 333 
a need for a greater specific gravity, in order to remain 
more easily at the bottom of the water; in other cases still, 
special protection against their enemies. 
The Testudinidae, or true land tortoises, are repre- 
sented in the Bridger beds by the genus Hadrianus. Three 
species have been recognized ; but one of these, Hadrianus 
allabiatus Cope, is based on very doubtful materials ; while 
Cope's H. octonarius is npt to be distinguished from Leidy's 
H. corsoni. This species reached a length of nearly three 
feet. No skull of the genus has yet been found, but there 
can be no doubt that it was a true land tortoise, such por- 
tions of the internal skeleton as have been discovered agree- 
ing well with those of Testudo. The shell had not yet 
reached that high degree of differentiation which charac- 
terizes that of our modern members of the genus Testudo, 
but it was on the way. This animal probably lived on the 
dry lands bordering the Bridger basin. 
The superfamily Trionychoidea, or soft-shelled tur- 
tles, was abundantly represented during Bridger times, 
somewhat more than one-half of the species found in south- 
western Wyoming belonging here. The writer is acquaint- 
ed with twenty-two species. According to the British 
Museum Catalogue of Chelonians there are at present living 
in the whole world only twenty-six species. Only six 
species now inhabit the United States. The Bridger rivers 
and brooks must have swarmed with these creatures. Some 
of them attained a large size, equalling or nearly so some of 
the Asiatic species, one of which, Amyda iriunguis (Forsk.) 
has the dorsal disk 31 inches long. 
Of the Trionychoidea the writer recognizes two fami- 
lies, the Trionychidse and the Plastomenidae. The type of 
the latter family is the genus Plastomenus Cope. That 
which distinguishes this genus from all the genera of the 
Trionychidse is the completeness of the plastron. In the 
members of the latter family there are always fontanelles 
between the lateral halves of the plastron, as well as others 
between the carapace and the outer ends of the hyoplastron 
and hypoplastron. In a few species these may become 
nearly closed in extreme old aqre. The species presenting 
the most nearly complete plastron is Cycloderma aubryi, 
