. Carnegie Institution of Washington publication. TRIONYCHIDtE. 501 half. Toward the midline the hypoplastron becomes much wider than the hvoplastron. The hypoplastron sends inward a finger-like process, which no doubt joined a similar process from the bone of the opposite side. The median border of both bones was very obliquely- beveled. The greatest thickness of the hypoplastron is 12 mm. The xiphiplastron present appears to belong to the right side. It has a central, thick portion, sculptured, oval, 25 mm. wide, and about 40 mm. long. This passes by a bevel into the thinner portion on all s
. Carnegie Institution of Washington publication. TRIONYCHIDtE. 501 half. Toward the midline the hypoplastron becomes much wider than the hvoplastron. The hypoplastron sends inward a finger-like process, which no doubt joined a similar process from the bone of the opposite side. The median border of both bones was very obliquely- beveled. The greatest thickness of the hypoplastron is 12 mm. The xiphiplastron present appears to belong to the right side. It has a central, thick portion, sculptured, oval, 25 mm. wide, and about 40 mm. long. This passes by a bevel into the thinner portion on all sides. The thinner part extended forward into digitations joining the hypoplastron, and posteriorly into others which connected with the corresponding bone of the opposite side. It is evident that there were large fontanels between the right and left halves of the plastron. The sculpture (plate 94, figs. 1-3) consists of a network of ridges which inclose pits ot rather regular form and size. The pits are of somewhat greater diameter than the ridges sur- rounding them. On the neurals there are about 5 pits in 12 mm., while on the hindermost costals there are only about 3 in the same distance. The pits increase somewhat in size from the proximal toward the distal ends of the costals. There is no tendency, near the free margins of the costals, toward either the disappearance of the ridges or toward their breaking up into tubercles. There are no welts on the surface such as we find in species of Plastomenus. The sculpture of the plastron is greatly like that of the carapace. The sculptured part ot the hvoplastron is nowhere more than 30 mm. wide, while that of the hypoplastron is 53 mm. Aspideretes singularis sp. nov. Figs. 656-662. The type of this species is a nearly complete specimen which was collected for Prof. E. D. Cope, by Mr. David Baldwin, in 1883, from the Torrejon beds of Chaco Canyon, San |uan County, New Mexico. It furnishes the oldest-known trionychid skull that is ac
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