242 FIELD COLUMBIAN MUSEUM. 
saurus, recently described* by Hatcher. These two genera display 
certain structural affinities common to no other North American 
group. The most important of these are: (1) the single median 
spines which persist throughout the vertebral column in the above- 
named genus and which appear to have been equally persistent in 
Brachiosaurus; (2) the unusual breadth of the sacrum in comparison 
with its length and the height of its component vertebra; (3) the 
simple structure of the anterior caudal vertebre; (4) the great expan- 
sion of the preacetabular portion of the ilium. In addition to these, 
are to be considered the presence of fourteen dorsal vertebre as 
described by Hatcher in Haplocanthosaurus, and the unusual length 
of the humerus in Brachtosaurus. Recent developments with regard 
to the number of vertebre in Dzplodocust and in Apatosaurust 
might throw some doubt upon this point were it not that the genera 
under consideration evidently belong to a distinct phylogenetic line, 
and that Hatcher has made a strong case in favor of fourteen 
dorsal vertebre in his genus. The vertebral formula in Brachto- 
saurus cannot, of course, be determined from the interrupted series 
preserved in the type specimen. It is most probable that the num- 
ber will prove to be the same as in Haplocanthosaurus, although the 
elongation of the dorsal centra eee suggest that a reduction had 
taken place in this form. 
Of the two genera, it will be Aad ed that Haplocanthosaurus is 
much the smaller and comparatively primitive, while Brachtosaurus 
is a long-limbed and highly specialized type. Points of generic differ- 
ence are abundant. The first and most evident of these is one of 
size, which is well indicated by the comparative length of femora. 
That of Haplocanthosaurus measures about fifty inches, while in 
Brachiosaurus the same bone measures fully eighty. More important 
differences are to be found in the relative height of the neural arches, 
the development of the hyposphene-hypantrum articulation, and the 
length of the centra in the dorsal vertebre. 
In the smaller genus the vertebral pedicles are peculiarly attenu- 
ate, giving to the neural areh the extraordinary elevation pointed out 
by Hatcher, and to the neural canal the unusually great vertical 
diameter. In the larger and more highly specialized Brachtosaurus 
there is no evidence whatever of this characteristic. The pedicles 
are broad antero-posteriorly, narrow and rounded on the posterior. 
margin instead of being produced into an angle, as in the smaller 
form, and the neural canal is but little deeper than wide. 
* Memoirs of the Carnegie Museum, vol. ii, No. 1. 
+ Memoirs of the Carnegie Museum, vol.i, No.1, p. 20. 
} This publication, GEoL. SErR., vol.ii, No. 4, p. 106. 
