THE CAMBRIDGE GREENSAND. 619 
very thin. The following vertebra, which is the eleventh of the 
whole series, is fully 2,4, inches long. The anterior facet for the 
chevron bone is not decomposed, and presents a flat oblique surface. 
The flattened ends of the centrum are slightly concave. The base, 
instead of being narrow and rounded, now begins to be defined by 
two slight parallel ridges. 
The twelfth vertebra is about 2} inches long. Here the neural 
arch is much more elongated, and the centrum less deep, so that the 
transverse-process ridge, which extends the whole length of the 
vertebra and is slightly convex in length, divides the side into two 
subequal regions, of which the lower is chiefly formed by the 
centrum, and the upper by the neural arch. The articular ends 
are subcircular, rather wider than deep, and moderately cupped 
concavely. ‘The articular face is about 1,2, inch in diameter. 
Several vertebree are here missing, and the remainder of the 
series from 13th to 19th have an elongated prismatic form. Those 
numbered 16 and 17 are the specimens which were brought to 
the Museum on a separate occasion from the others. All these 
vertebree agree in every thing except length, relative development of 
the facet for the chevron bones, and preservation. All have the ar- 
ticular face of the centrum subcircular and concavely cupped. The 
sides are divided by a median ridge into two areas on each side, of 
which the upper pair are rather the smaller and more deeply exca- 
vated. At the base there is a narrow, slightly channelled region 
defined by parallel sides ; and a similar region, somewhat wider and 
shorter, runs along the upper surface of the neural arch, so that the 
vertebra have a compressed hexagonal aspect. The 13th is rather 
over 33 inches long; the 14th 2,8; inches long. The 15th (imperfect) 
shows the anterior end of the neural arch, but is not sufficiently 
well preserved to show whether the zygapophyses interlocked. 
The 16th vertebra (fig. 5, p. 620) is nearly 3,4 inches long, and is the 
best preserved inthisregion. It has the anterior face of the centrum 
subhexagonal, but with the upper pair of lateral elements shorter 
than the lower pair, 1} inch wide at the outer angle, and about as 
deep to the rounded surface, where a chevron bone may still have been 
attached. The posterior articular face is of the same size. The 
extreme depth of the centrum in front is 1,4, inch. The median 
ridge on the side of the centrum is concave in length, so that the 
diameter of the bone in the middle of the ridge is l inch. The 
lateral spaces above and below this ridge are 55, of an inch high in 
the middle of the centrum; the narrow base is less than 7 inch 
wide. The width of the neural arch posteriorly is about 3 inch; 
along it in the superior median line runs a slight ridge. The neural 
arch has a more pinched aspect than the part of the centrum below 
the median lateral ridges. The 17th vertebra is 22 inches long; it 
shows that the cheyron bones still exist, but that the articular faces of 
the centrum have become slightly smaller, though hardly more deeply 
cupped. The 18th and 19th vertebra are represented by fragments 
which add nothing to our knowledge beyond showing that the 
central part of the centrum was either hollow or occupied by a 
