380 Emerson and Zoomis — Stegomus Zongipes. 



suclius and Erpetosaurus of Newton* also have some features 

 resembling S. longipes but are far wider differentiated than the 

 German genus. Aetosaurus has about twenty-five sets of plates 

 from the head to the pelvis, each consisting of a median pair of 

 large scutes and small quadrate scutes outside these. Stegomus 

 has about twenty-eight exactly similar sets of plates. 



In the skull, however, there are marked contrasts. The orbit 

 of both genera is bounded above by extra supraorbital bones,, 

 but the orbit of the Aetosaurus is further back than that of 

 Stegomus : the result of which is that the former has only a 

 single small supra-temporal vacuity, while the latter has at 

 least a very large vacuity, and possibly that divided into a supra- 

 temporal and a lateral temporal vacuity. The Stegomus has 

 a wider skull and above the orbit a vacuity or at least a deep 

 depression. The vertebras of Aetosaurus are procoelous, while 

 those of Stegomus are amphicoelous. Ornithosuchus has 

 platycoelous vertebral centra. The sacrum in the other known 

 Aetosauridae includes three vertebras, but in Stegomus only two 

 are united to the ilium. Both the fore and hind limbs of 

 Stegomus are much longer and more slender than the Aeto- 

 saurus, this being the character which has suggested the spe- 

 cific name longipes. 



The features above described show this fossil to be the 

 remains of a small armoured lizard-like creature, with long legs. 

 It seems to be a land form and of extreme agility. 



The following are the measurements of the principal parts : 



j mm 



Length from snout to root of tail . 149 r 



Length of skull 35 



Breadth of skull in occipital region 27 



(29 allowing for fracture) 



Depth of skull in quadrate region 11 



Median Plate 2 8 mm transversely by 4-J longitudinally 



" Plate 3... 5 J " " 2 " 



" Plate 4... 5 " " 2^ " 



" Plate 5... 4^ " " 5 



The succeeding plates gradually increase transversely up to 

 10 mm at about the middle of the body and then slowly diminish 

 again. All are drawn to scale in figure 2, PI. XXII. 



The vertebral centra in front of the pelvis are 3j- mm long by 

 3 mm wide. Caudal vertebrae 3 mm long by 2 mm wide. The trans- 

 verse process of the last vertebrae are 6 mm long. 



EXPLANATION OF PLATE XXII. 



Figure 1. — Stegomus Longipes, photograph natural size. 



Figure 2. — Stegomus Longipes, outline drawing to show details, dotted 



lines indicating the portions reconstructed. 

 Figure 3. — Stegomus Longipes, side view of skull reconstructed. 



Amherst College, Mar. 1, 1904. 



* Newton E. T. 1894, Phil. Trans., vol. clxxxv, Reptiles of Elgin Sand- 

 stones. 





