hatcher: diplodocus (marsh) 27 



ence to the measurements. The prezygapophyses extend beyond the anterior ends 

 of the centra, while the posterior zygapophyses do not extend beyond the centra. 

 See Plates YI. and VII. 



The Third Dorsal. — Paired spines broad at base, but with pointed and widely 

 separated extremities presenting elongated external rugosities. These spines, as those 

 of the preceding dorsals and cervicals, are formed by the union of the superior blades 

 of the post- and prezygapophysial laminae. The latter in this vertebra function as 

 the superior blades of the diapophysial laminae, which are rudimentary in D. 4 and 

 obsolete in D. 3. The post- and prezygapophyses are much elevated, as are also the 

 transverse processes. The superior surfaces of these processes are almost on a line 

 with the zygapophyses. Thus the anterior and posterior blades of the horizontal 

 laminse in this and the succeeding dorsals occupy approximately the same plane. 

 There commences in this vertebra an approximation to the diplosphenal articula- 

 tions which obtain in the posterior dorsals. The support for the postzygapophyses 

 inferiorly is much modified. The inferior blades of the postzygapophysial laminae 

 instead of converging and intersecting, as in the succeeding vertebras, are each divided 

 into two branches, one vertical, the other horizontal. The former descend directly 

 from the inner sides of the zygapophyses in sharp parallel laminae to the outer walls 

 of the neural canal ; the latter converge and meet in the middle line a little below the 

 zygapophyses, and form a long, very delicate, single median lamina, which descends, 

 parallel to and midway between the vertical branches, to the superior border of the 

 neural canal, where it again divides and sends off right and left laminas to meet the 

 inferior extremities of the vertical blades and form the superior border of the neural 

 canal. In this manner these laminae enclose and bisect longitudinally an oblong 

 with concave extremities, thus 1 1 1 , as shown in PI. IV., Fig. 3. The transverse 

 processes are widely expanded, with their extremities turned downward and back- 

 ward and presenting two rugosities. The one larger and superior and external is 

 evidently for muscular attachment. The other smaller and inferior and external is 

 the capitular rib facet, and looks downward, outward and backward, instead of 

 directly downward as in Ds. 1 and 2. The inferior blade of the diapophysial lamina 

 is more perpendicular than in the preceding vertebrae, but is still quite oblique. The 

 inferior branch of the anterior blade of the horizontal lamina is still widely separated 

 from the superior branch. The anterior zygapophyses are supported inferiorly by 

 very powerful laminae, but the inferior blades of the secondary intersecting lamina 

 are wanting. The pleuro-central cavity is deep and is invaded by the capitular facet, 

 which is supported inferiorly by a short pleuro-central lamina which bisects the 

 pleuro-central cavity. Centrum strongly o|)isthocoelous. The external wall is 



