maesh.] DIRACODON. 193 



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. L, figs. 4, 5, and 6. 



The position of these caudal spines with reference to the tail is indi- 

 cated in the specimen figured on PI. LI, which shows the vertebra?, 

 spines, and plate as found. 



DIRACODON. 



The American genera of the Stegosaurra are Stegosaurus and Dirac- 

 odon. Of the former there are several well-marked species besides 

 S armatus, the type. Of the latter genus but one is known at present, 

 Dhacodon laticeps, the remains of which have hitherto been found at a 

 single locality only, where several individuals referred to this species 

 have been discovered. Aside from the form of the skull, these speci- 

 mens have in the fore foot the intermedian and ulnar bones separate, 

 while in Stegosaurus these carpals are firmly coossified. 



All the American Stegosaurida? have the second row of carpals 

 unossified and five digits in the manus. In the hind foot the astrag- 

 alus is always coossified 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 kuown 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 metatarsals, which are 

 short and massive, but longer and much larger than the metacarpals. 



In one large specimen, of which the posterior half of the skeleton was 

 secured, no trace of dermal armor of any kind was found. If present 

 during life, as indicated by the massive spines of the vertebra?, 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 coossified with the sacrum, thus form- 

 ing a strong bony roof over the pelvic region, as in birds. 



This specimen represents a distinct species, Stegosaurus duplex. It 

 was originally referred by the writer to S. ungulatus. In the sacrum 

 of this species each vertebra supports its own transverse process, or 

 rib. as in the Sauropoda, while in iS'. ungulatus the sacral ribs have 

 shifted somewhat forward, so that they touch, also, the vertebra in 

 front, thus showing an approach to some of the Ornithopoda. 



CHARACTERS OF STEGOSATJRIA. 



The large number of specimens of the Stegosauria now known from 



the American Jurassic, and the fine preservation of some of the remains, 



aid in forming a more accurate estimate of the relations of the group 



to the other dinosaurs than has hitherto been possible. The presence 



16 geol, PT 1 13 



