48 
EOCENE TESTUDINATA. 
variously distributed, are often quite elegant. The species do not attain the 
average size of the Trionyches of the same era; but the P. communis, P. 
laclirymalis, and P. muUifoveatus exceed in dimensions the living species of 
North American waters. 
The species above named, in which the sternal characters are evident, 
are the only ones which can certainly be referred to the genus; but several 
others from the Eocene beds can with much probability be referred here 
also, the whole number being eight. Four species from the Fort Union 
Cretaceous beds have been referred to Plastomenus, but, as already remarked, 
as a provisional arrangement until their structure is better known. The 
P. tJiomasii is also of uncertain reference to this genus. 
I. Surface without ^velts, or with the sculpture thrown into ridges; 
a. No ridge-lines: 
Surface with sharp, fine wrinkles.P. corrugatiis. 
Surface with more remote wrinkles, little inosculating.P. trionyclioides. 
Surface honeycombed with thick, inosculating ridges .. P. muUifoveatus 
aa. Sculpture thrown into ridges : 
Surface coarsely honeycombed with fine ridges... P.fractus. 
II. Sculpture interrupted with solid welts; pits small or reduced to punctse: 
Surface with transverse ribs separated by one or two rows of pits..P. serialis. 
Welts few, oblique, separated by numerous pits - -.. P. communis. 
Welts broken up into short ridges behind; intervening surface 
punctate... .......P. laclirymalis. 
W’elts represented posteriorly by tubercles separated by smooth 
surface; anteriorly unbroken, the surface punctate----P. cedemius. 
Of these species, P. corrugatus, P. muUifoveatus, P. fractus, P. serialis, 
P. communis, and P. laclirymalis have been found in the Wahsatch beds of 
New Mexico; and the P. trionyclioides, P. muUifoveatus, and P. cedemius in 
the Bridger beds of Wyoming. 
I .—Carapace with ridges which inosculate less or more: 
Plastomenus corrugatus, Cope. 
Plate sxv, figs. 20-26. 
Catal. of Eocene Vert, of New Mexico, U. S. Geog. Survs. W. of 100th M., 1875, p. 35. 
Ridges fine, inosculating less; no welts. 
Remains of six individuals are provisionally referred to this species; 
