STRUCTURE AND RELATIONSHIPS OF OPISTHOCGLIAN DINOSAURS. 231 
DORSAL VERTEBR-. 
The seven dorsal vertebre preserved in the specimen were found 
in a series, and but little displaced from their relative positions. 
The centra and transverse processes are considerably distorted by 
the compression to which they have been subjected. These distor- 
tions have been corrected in the specimen as far as was practicable, 
but no effort has been made to further correct them in the drawings. 
(Plate LX XII.) The vertebre are distinguished by the lightly con- 
structed and elongate centrum with its large lateral cavity, and by 
the single neural spines, short in the posterior members of the series, 
but becoming more and more elongate anteriorly. Equally distinctive 
is the unusual development of the hyposphene-hypantrum articu- 
lation. The whole structure suggests lightness and flexibility 
attained with an evident sacrifice of that strength which is every- 
where apparent in the unwieldy Apatosaurus. 
The number of vertebrz in the dorsal series cannot, of course, be 
determined from this specimen. Reasoning from certain similarities 
between this genus and Haplocanthosaurus, in which the number has 
been determined as fourteen,* we may expect a more numerous 
series than characterizes Apatosaurus and Diplodocus. As this num- 
ber must for the present remain conjectural, however, the vertebrz 
will be referred to as presacral and numbered from the sacrum 
forward. 
The first presacral vertebra may be distinguished, as in all opis- 
thoccelians so far as the writer has observed, by the massive post- 
zygapophyses which overhang the posterior end of the centrum, by 
the low stout spine and the short centrum with small lateral cavities 
or pleurantra. From this point forward, both the centra and spines 
rapidly elongate, the diapophyses become more and more expanded, 
but, contrary to all that might be expected, the zygapophyses be- 
come reduced almost to insignificance. 
The centrum in the dorsal vertebre is opisthoccelous in type, but 
less pronouncedly so in the anterior members of the series than is 
common in the mid-dorsal region of most forms. In the first pre- 
sacral the centrum is similar in length to that of Apatosaurus, but in 
' the preceding vertebre it rapidly increases in length. The pleuran- 
trum is of moderate size; the anterior end of the centrum is truncate, 
with a slight convexity above the middle: the posterior end, now 
badly distorted, was doubtless uniformly concave. The second cen- 
* Memoirs of the Carnegie Museum, vol. 2, No. 1. 
+In the preliminary description of this genus the writer, estimating from the specimens stil] 
in the matrix, characterized the posterior dorsal centra as longer than wide. Closer examination 
shows that this is not true of the last dorsal. 
