APRAOTOCLEIDUS TERETIPES. 



615 



contributed by the neural arch, and the lower to the portion contributed by the 

 centrum. The vertebrae posterior to the first sacral have not been preserved. 



The ribs (text-fig. 6) in cervical regions have been referred to above. In anterior 

 dorsal regions the ribs are short (B, C), but increase rapidly in length (D) towards 

 middle dorsal regions. The heads (h.) of the ribs are smooth and concave, and fit 

 the rib facet on the corresponding transverse process with great accuracy. The 

 upper anterior margin of the articular surface is broken by a small lip sharply folded 

 back. In middle dorsal regions this lip becomes less abrupt and the articular surface 

 of the head becomes almost flat. The head (h.) bears numerous roughened pits and 

 tubercles (s.) for muscle attachment, and these are most pronounced on the upper 

 and posterior portions of the head. The rugosities continue down the anterior side 



n.S 



Text-Fig. 5. 



Sacral vertebra and rib of Apractodciclus teretipes : A, from right side ; B, from behind ; 0, sacral rib. (Type specimen 

 V. 1091, £ nat. size.) a.z., anterior zygapophysis ; n.c, neural canal; n.s., neural spine ; p.z., posterior zygapo- 

 physis (broken) ; ■/•„/., rib facet ; s.f., facet for ilium. 



of the rib and converge towards the smooth ridge (r.), which is most marked in the 

 ribs of anterior dorsal regions. In the long ribs (D) of middle dorsal regions this 

 ridge is a much less prominent feature. These long heavy ribs contain a well-defined 

 pit (p.) a short distance from the end, on the upper anterior side. This pit probably 

 served for the insertion of a strong ligament for the support of the long and 

 heavy rib. The lower portions of the long ribs, below the ridge (r.), remain of almost 

 uniform diameter and oval to circular in section. At the distal end there is a slight 

 expansion, and the bone ends in a concave, oval surface, having been tipped with 

 cartilage in life. The sacral ribs (text-fig. 5, C) are short strong rods of bone some- 

 what expanded at the facets at their ends. The facet (s.f.) for the attachment to the 

 ilium is an irregularly convex surface with well-defined margins but no prominent 

 tubercles. Numerous ventral ribs and rib fragments are present, # and doubtless these 

 went to the formation of a plastron of ventral ribs (text-fig. 9) arranged as in 

 Cryptodeidus (Andrews, 1910, p. 175, text-fig. 86). 



