The Fossil Turtles of the Bridger Basin— Hay. 335 
The Trionychidae form an important part of the exist- 
ing faunas of North America, Africa, and Asia. The 
division of the family into genera has proved to be a diiffi- 
cult undertaking. Most of the living, as well as of the 
fossil species, have hitherto been referred to the genus 
Trionyx. The genera have been discussed by the writer 
in the Proceedings of the American Philosophical Society, 
vol. xlii, 1903, pp. 268-274. Dr. Stejneger (Science, xvi, 1905, 
p. 228) has since shown that Amyda Oken antedates Aspi- 
donectes Wagler. The new genus Aspideretes is proposed 
by the present writer in the article cited for such Trioriy- 
chidge as possess eight pairs of costal plates and a preneural, 
a plate between the nuchal and the first neural. To this 
genus are referred three Bridger species. One of these was 
described by Dr. Leidy under the name Trionyx guttaius 
His material consisted of only a portion of the rear of the 
carapace. The expedition of 1903 secured portions of three 
individuals. These furnish most of the plastron and of the 
carapace, the pelvis, and portions of the femora. I^'nfortu- 
nately, in all the specimens the region in front of the first 
neural is damaged, so that the existence of the preneural is 
yet doubtful, but probable. Even in aged individuals there 
was a pair of fontanelles, one on each side of the midline, 
behind the nuchal. Two other species of the genus were 
discovered in 1903, both represented by fine carapaces. 
The genus Amyda Oken embraces species which have 
