Stegomurus priscus, sp>. nor. 113 



weight of the double row of dorsal plates. It is evident that a rib 

 fixed to the parapophysis and diapophysis in the Stegosaurian way 

 was much more fitted for supporting weight than one in which the 

 two points of attachment were nearer to each other and more on 

 a horizontal plane, with the rib directed nearly at right angles to the 

 vertebral column. 



To illustrate this the diagrams Fig. 2a and 2b were drawn according 

 to the evidence afforded by 0. armatus and St. ungidatus. By breaking 

 up in these figures the vertical pressure of the dermal armour on the 

 rib 7 into its components a («') and ft (/3'), it becomes clear that in 

 St. ungidatus (Fig. 2a) the strain (ft) on the diapophysis (A) is by 

 m just about half as strong as in 0. armatus (Fig. 2b, ft') while the 



pressure a (a) on the parapophysis (p) is augmented, for a = — - and 



ft' is larger than ft. In animals where the dermal armour becomes 

 supported by the vertebral column and not only by the rib, as in 

 crocodiles, tortoises, Ancylosauridae, and some others, such an 

 elevation of the diapophysis need not be developed. 



Even the strong downward curve of the rib beyond its culminating 

 point, reducing as it does the transverse diameter of the body-cavity, 

 is probably more due to the pressure of the dermal armour than to 

 anything else, for it is evident that with a rib curved strongly 

 downward the dilatation of the body-cavity, as necessary for 

 breathing, can be easily brought about by contraction of the muse, 

 levator costarum without shifting the weight of the dermal armour, 

 while this would be impossible if the distal ends of the long ribs were 

 not directed downward but outward. 



Since Stegosaurus was probably a cold-blooded animal, the increase 

 of body-surface and greater loss of heat as resulting from the lateral 

 Compression- of the body would, of course, in this animal not be of 

 so great importance as in a warm-blooded animal, where there is 

 a minimum of body- surface united with greatest capacity for the 

 respiratory organs ; thus a more rounded body-section is needed. 



Two other quite marked features of the dorsal vertebrae of St. priscus, 

 in which this animal makes a close approach to St. ungidatus, are the 

 lateral compression of the neural canal and the absence of cavities on 

 the sides of the centrum. The shape of the transverse and longitudinal 

 sections of the plano-concave centra seem, however, otherwise to be the 

 same in both Omosaurs and Stegosaurs. 



A good view of a middle dorsal vertebra is given in Fig. \c, which 

 is based principally on one bone, but has both zygapophyses restored 

 on evidence afforded by another specimen. 



The sacral vertebras are all wanting in St. priscus, but the proximal 

 middle and distal caudals are each represented by numerous pieces. 



The anterior caudals (Figs, oa, b) are of the same type as in 

 Omosaurus, the chief difference being only that the tubercle on the 

 superior border of the dilatated costoid is somewhat less developed in 

 the former than in the latter. On these strongly abbreviated vertebrae 

 no chevron bones seem to have articulated. The proximal caudals of 

 St. priscus are easily distinguished from St. ungidatus by the weaker 

 development of the neural spine and the costoids. 



DECADE V. — VOL. Vni. — NO. III. 8 



