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: /A.os, a nail used for ornament; -n-avo-n-Xia, full armor. 



Type: Helopanoplia distincta Hay. 



This genus differs from Tnonyx (EmyJa Gray) in having the hyoplastron free from the 

 hypoplastron until advanced age. In Trionyx 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 is 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 

 10 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 9 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 Chelomans. 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 Trionychidae 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 A. hurum 

 and A, leithii. Trionyx melitensis described by Dr. Lydekker from the Miocene of Malta must 

 be referred to the same genus. 



That which distinguishes this genus from Amyda 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- 

 nychidae. This bone is present in the Plastomenidae also and in some of the Baenidae. 



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., A . 

 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. 



