14 Prof. O. C. Marsh—On Stegosaurus. 
shown on PI. IJ. Figs. 2 and 3, In some other forms there were 
three pairs, and in S. stenops but two pairs have, as yet, been found. 
In one large species, Stegosaurus sulcatus, there is at present 
evidence of only one pair of spines. These are the most massive of 
any yet found, and have two deep grooves on the inner face, which 
distinguish them at once from all others known. One of these 
grooved spines is represented on PI. II. Figs. 4, 5, and 6. 
The position of these caudal spines with reference to the tail is 
indicated in the specimen figured on Pl. III., which shows the 
vertebra, spines, and plate, as found. 
The American genera of the Stegosauria now known are Stegosaurus, 
Hypsirhophus and Diracodon. Of the former there are several well- 
marked species besides S. armatus, the type. Of the latter genus 
but one is known at present, Diracodon laticeps, the remains of 
which have hitherto been found in Wyoming at a single locality 
only, where several individuals referred to this species have been 
discovered. Aside from the form of the skull, these specimens 
have in the fore foot the intermedian and ulnar bones separate, 
while in Stegosaurus these carpals are firmly codssified. 
All the known American forms appear to have the second row of 
carpals unossified, and five digits in the manus. In the hind foot, 
the astragalus is always codéssified with the tibia, even in very young 
specimens, while the calcaneum is sometimes free. The second row 
of tarsals is not ossified in any of the known specimens. Only four 
digits in the hind foot are known with certainty, and one of these 
is quite small. All forms have at least three well-developed meta- 
tarsals, which are short and massive, but longer and much larger 
than the metacarpals. Most of the bones originally referred to the 
hind foot of Stegosaurus ungulatus, and figured as such (Amer. Journ. 
Sci. vol. xxi. pl. viil.), although found with the posterior extremities, 
subsequently proved to belong to the fore foot of another larger 
species. 
In one large specimen, of which the posterior half of the skeleton 
was secured, no trace of dermal armour of any kind was found. If 
present during life, as indicated by the massive spines of the ver- 
tebree, it is difficult. to account for its absence when the remains 
were found, unless, indeed, the dermal covering had been removed 
after the death of the animal, and previous to the entombment of 
the skeleton where found. In this animal, the ilia were firmly 
codssified with the sacrum, thus forming a strong bony roof over the 
pelvic region, as in birds. 
This specimen represents a distinct species, which the writer has 
pamed Stegosaurus duplex. It was originally referred by him to 
S. ungulatus, and the pelvic arch was figured under that name.’ In 
the sacrum of this species, each vertebra supports its own transverse 
process, as in the Sauropoda, while in S. ungulatus these processes 
have shifted somewhat forward, so that they touch, also, the vertebrz 
in front, thus showing an approach to some of the Ornithopoda. 
The great weight of the armour in Stegosaurus, taken in connection 
1 Amer. Journ. Sci, vol. xxi. pl. vii. Feb. 1881. 
