66 FOSSIL TURTLES OF NORTH AMERICA. 
4 
the Trionychoidea as one of three suborders of the Testudines, the other two being the Athece 
and the Thecophora. Further consideration of the subject has convinct him that the group 
is too closely related to the Cryptodira to justify such a wide separation of the two. On the 
other hand, there are structural differences of too great value to permit the trionychids to be 
reduced to the status of one or two families under che Cryptodira. It is now believed that their 
relationships to other turtles will be correctly exprest by making them a superfamily of the 
Thecophora, the other superfamilies being the Amphichely dia, the Cryptodira, and the 
Pleurodira. This classification is that adopted by Dr. Boulenger in his Catalogue of Chelon- 
ians, etc., in the British Museum. 
For a discussion of the structure and relationships of the trionychids the reader is referred 
to Baur’s papers in the Zoologischer Anzeiger, vol. x, 1887, p. 96; vol. x1, 1888, p. 7363 vol. x11, 
1889, pp. 241-243, and American Naturalist, vol. xXxtv, 1870, p. 530; Dr. Boulenger’s catalog 
referred to above, and to a paper by the present writer (Bull. Amer. Mus. Nat. Hist:, x1, 
1905, pp- 137~175). For a discussion of Haeckel’s view that the Trionychoidea are the most 
primitive of ihe Thecophora the reader is referred to page 25 of the present work. 
The geographical distribution of the living Trionychidz is discust on page 35 and is 
illustrated by fig. 16. 
Family PLASTOMENID Hay. 
Trionychoidea with skull like that of the Trionychide. Neck unknown, but probably 
like that of the Trionychide. No peripheral bones. Epiplastra separated from the hyoplastra 
by the large, crescentic entoplastron. Hyoplastra, hypoplastra, and xiphiplastra closely united, 
as in the Emydide. Feet unknown. 
the genus Plastomenus Cope. ‘The eeeichite of f this, so far as Neeneeas has been ea 
from the Bridger species. The cervical vertebra and the feet are unknown. 
The Plastomenide differ from the Trionychide, so far as known, principally in the 
structure of the plastron. In the latter family the ventral armor differs much from that of the 
Cryptodira. The bones have suffered reduction, so that great fontanels intervene between the 
bones of the opposite sides and between the entoplastron and the hypoplastra. The ento- 
plastron 1s V- shaped and excludes the epiplastra from the hypoplastra. In the Plastomenidz 
the three hinder pairs of bones join their fellows closely at the midline, as in the Cryptodira, 
and the hypoplastra are closely sutured to the hyoplastra and to the xiphiplastra. There 
appears to have been little space between the entoplastron and the hypoplastra. Indeed, the 
plastron is more completely closed than in the Chelydride and Cheloniide. Furthermore, 
the entoplastron, if we may form our conclusion from Plastomenus edemius, was broad fore 
and aft and elongate kidney -shaped. We do not know what were the form and connections of 
the epiplastra. 
A peculiarity of structure displayed by some species of the family is the great development 
of the eighth pair of costals. In most species of Trionychide the eighth Goutal are greatly 
reduced ands in some cases wholly wanting. In Plastomenus thomast and some other species 
this pair of costals is greatly developt. The character does not, however, appear to be shared 
by all the species. 
Genus PLASTOMENUS Cope. 
At the present time the generic characters of Plastomenus can not be separated from those 
assigned above to the family. So far as known there is present a preneural bone, and some of 
the hinder neurals are wanting. 
Type: Plastomenus thomas Cope. 
Professor Cope’s first definition of the genus Plastomenus (Proc. Acad. Nat. Sci. Phila. 
1873, p. 278) is hardly sufficient to distinguish it from other trionychids. For a reason which 
the present writer does not understand he concluded that there was present ““a nuchal marginal 
which does not exist in Trionyx,” but no such bone has been shown to have existed. He was 
