888.] ANATOMY OF THE MESOSUCHIA. 419 



In immature individuals all the component elements of the atlas 

 are distinct, but in mature individuals they are often synostosed, as 

 are also the atlas and epistropheus. 



Basilar piece (Stannius). — In its form and its connections this 

 part agrees closely with that of extant Crocodilians. Its anterior or 

 cranial surface contributes nearly the lower or ventral half of the 

 articular cup for the reception of the occipital condyle. Its inferior 

 surface is convex transversely ; whilst its superior is slightly concave 

 in this direction, and it is adapted to the corresponding surface of the 

 pars odontoidea. Its supero-lateral margins unite with the " lateral 

 pieces." Its posterior margin, thin, has at its junction with the 

 lateral margin, on each side, a large articular facet for the first pair 

 of ribs. 



" Lateral pieces." — These are composed of a thin, compressed, 

 upper part which forms the side-wall of the neural canal, and of a 

 stouter lower half. The division between these two parts is 

 indicated on the median surface by a slight horizontal ridge which 

 marks the former attachment of the " transverse ligament." The 

 anterior border of the stouter lower part is so wide that it deserves 

 the term surface. Smooth, articular, forming a small segment of a 

 circle, it contributes the upper lateral border of the occipital cup. 

 The inferior border of the lateral piece unites with the supero- 

 lateral border of the " basilar piece." The posterior border, and the 

 upper border of the upper part of the lateral piece, that part which 

 bounds the neural canal, are thin ; and at their junction they are 

 produced backwards, and they form a rudimentary post-zygapophysis 

 which articulates with a similarly dwarfed prse-zygapophysis on the 

 epistropheus. The outer surface of the " lateral piece " is traversed 

 obliquely by a ridge, which, starting from the angle formed by the 

 junction of the anterior and superior margins of that part of the bone 

 which bounds the neural canal, descends in a backward direction 

 towards the postero-inferior angle, where it ends in a small projection 

 or tubercle situated in the level of the diapophysis on the epistro- 

 pheus. For reasons presently stated this little tubercle should rank 

 as an upper atlantal transverse process or diapophysis. The median 

 aspect of the stouter, lower part of the lateral piece rests on the 

 pars odontoidea. 



Pars odontoidea. — This has a slightly skewed cubic or pyramidal 

 form, its inferior or ventral part being slightly smaller than the 

 upper. The posterior surface, plane, is marked by horizontal 

 ridges and furrows indicative of synchondrosis with the cranial, 

 terminal surface of the centrum of the epistropheus. In aged 

 individuals it is frequently synostosed with this. The outline of this 

 (posterior) surface of the pars odontoidea is an inequilateral four- 

 sided figure, iu which the upper is longer than the lower side 

 Upon its upper surface may be discerned (1) a relatively wide, 

 smooth, median tract — the floor of the neural canal ; this is slightly 

 encroached upon laterally by (2) a rough synchondrosial impression, 

 marking the attachment of the neurapophysis, which also descends 

 upon the lateral surface. The postero-lateral angles of the upper 



