TRIONYCHID. 485 
Genus HELOPANOPLIA nov. 
Trionychids with hyoplastron and hypoplastron not co-ossified, and with both the carapace 
and the plastron ornamented with numerous tubercles which resemble the rounded heads of 
small nails. The tubercles in some places coalescing to form short ridges. 
Derivation of name: #Aos, a nail used for ornament; zavord(a, full armor. 
Type: Helopanoplia distincta Hay. 
This genus differs from Trionyx (Emyda Gray) in having the hyoplastron free from the 
hypoplastron until advanced age. In Trronyx the two bones co-ossify at a very early stage. 
Doubtless better materials of the fossil species will reveal other differences. 
Helopanoplia distincta sp. nov. 
Plate 88, figs. 4, 5- 
The only known materials which indicate the former existence of this species are a frag- 
ment of one costal plate and a portion of one hypoplastron or hyoplastron. These fragments 
were collected by Prof. J. B. Hatcher, while in the employ of Prof. O. C. Marsh, at that 
time vertebrate paleontologist of the U. S. Geological Survey. The remains were secured in 
the Laramie deposits of Lance Creek, Converse County, Wyoming, and indicate a rather 
large and heavy soft-shelled tortoise. The fragment of carapace (plate 88, fig. 4) is a part of 
the upper half of a costal plate. It 1s only 34 mm. long and does not extend entirely across the 
plate, but apparently lacks little of doing so, since the thickening of the rib lies rather closer 
to the side bearing the suture than to the other. The thickness thru the sutural border is 
to mm.; thru the rib, 13.5 mm. at the proximal end of the bone, and 10.5 mm. at the distal end. 
Whether the fragment of the plastron (plate 88, fig. 5) belongs to the hyoplastron or to the 
hypoplastron can not be determined with certainty; it seems to belong to the latter of the left 
side. The piece formed a portion of the plastron between the fore and hind legs. The bone 
is thickened most at the end farthest from the midline, being there 15 mm. ‘Toward the mid- 
line the thickness becomes reduced to g mm. At the free border the bone is suddenly reduced 
in thickness and the edge is subacute. 
The ornamentation of this species resembles much that of Trionyx granosus, called Emyda 
granosa, in Boulenger’s Catalogue of Chelonians. The costal plate is thickly studded with small, 
rounded tubercles which look like the heads of insect pins. Four or five of these occupy a dis- 
tance of 5 mm. The distance between the tubercles is, on an average, somewhat less than the 
diameters of the tubercles themselves. Perhaps half of the tubercles become connected in 
twos and threes to form short and winding ridges. Similarly, but to a greater extent, the 
tubercles of Trionyx granosus coalesce along the middle of the back. 
On the piece of plastron the tubercles are less crowded and seldom join to form ridges. 
It is believed that this style of ornamentation is so different from that of the other species 
of Trionychide that a distinct genus is indicated. 
Genus ASPIDERETES Hay. 
Carapace with 8 pairs of costal plates; one or more of the posterior pairs in contact on the 
midline. A preneural plate between the nuchal and the true first neural. Young of living 
species with numerous longitudinal dorsal dermal ridges or series of tubercles. 
The type of this genus is Trionyx gangeticus of Cuvier. Other living species are 4. hurum 
and A. leith. Trionyx melitensis described by Dr. Lydekker from the Miocene of Malta must 
be referred to the same genus. 
That which distinguishes this genus from dmyda is the presence of a preneural bone. The 
writer regards this as a primitive character which has been lost by the other genera of Trio- 
nychide. This bone is present in the Plastomenidz also and in some of the Baénide. 
It is a noteworthy fact that no trionychid from the Cretaceous deposits is known to be 
without this bone. Two Upper Cretaceous species are arranged under Amyda, viz., 4. 
prisca and A. halophila, but these are based on fragments only and it is quite probable that 
better materials would show the presence of the preneural. 
