18 



W. LlLLJEBORG, 



upwards, somewhat depressed, with a round point and concave upper side, 

 and with the outer end (b) almost round. This last mentioned end is not 

 larger than the inner, which has on the under side a large ohiong oval 

 tuberosity somewhat projecting at the back edge of the bone, though not 

 visible on the side exhibited in the figure. The front border of the bone 

 is a little within the centre [e] sharp, and the back border a little without 

 the centre has a projecting tuberosity, and the middle portion of the bone 

 is somewhat flattened. 



The atlas (PI. I. figg. 6 and 7) is very strong and thick, and in 

 form corresponds most nearly with that of the Jfec/aptera, thoug it is 

 still thicker, and deviates remarkably from that type in having a di- 

 stinct hypapophysis (Retzius) (a), pointing backwards , and accordingly 

 most projecting at the back part of the bone. (Fig- 7 a). It also 

 differs in that the foramen spinale is broad at the upper part and forms 

 an acute angle at the lower. It is somewhat oblique on account of the left 

 lateral process going higher up than the right, and the articular surface for 

 the left condylus occipitalis is larger than for the right. The fossae condy- 

 loideae, by carrying their upper and inner border a little inward, divide 

 the foramen spinale into an upper and lower division. The lateral proces- 

 ses are short, compressed, blunt, and knotty, and their height considera- 

 bly greater than their length. Their direction is somewhat oblique from 

 the lower part backward, and they are situated above the middle of the 

 sides of the bone x ) and directed obliquely upwards and outwards. Between 

 them and the upper edge of the fossae condyloideae, and nearer the latter, 

 is a foramen for the arteria vertebralis, which pierces in an inward direc- 

 tion through the base of the arcus (neurapophyses Owen). This foramen 

 is shown in fig. 7 (b). The two fossae condyloideae or the concave arti- 

 cular surfaces for articulation with the condyli occipitales are separated at 

 the lower extremity by a shallow furrow or sulcus, and extend with their 

 lower border beneath the edge of the lower arcus, so that this latter is in 

 its middle part hollowed out. The hypapophysis (a) is a blunt tubercle 

 pointing backwards, its point extending beyond the level of the vertebra, 

 and it was doubtless received in a corresponding fossa on the lower side 

 and anterior border of the epistrophaeus. On the upper side of the upper 

 arcus is a rudimentary processus spinosus or neural spine. When the atlas 



') In this respect this bone agrees with the corresponding hone of the Megap- - 

 ftaptcra, but differs from that of the Sibbaldlns, in which genus the lateral processes 

 also have the short and compressed form, but project from the middle of the sides. 



