232 FIELD COLUMBIAN MUSEUM. 
trum takes on a more typical opisthoccelous outline. From this 
point forward the centra increase slightly in length, the pleuraccele 
enlarges, and the general structure displays even greater lightness. 
A cross-section shows an internal arrangement very similar to that of 
Apatosaurus (Brontosaurus) as figured by Marsh. 
However, the lateral cavities are larger, and the peripheral walls 
thinner than have been observed by the writer in that genus. The 
median septum is so fragile as to be lost entirely in some of the 
vertebre. 
The neural arch is unusually slight in this genus. Its posterior 
surface is narrow and rounded; anteriorly it is produced into two 
laterally directed ridges which descend from the base of the prezyga- 
pophyses to the anterior rim of the centrum. Inclosed by these ridges 
and by the buttresses supporting the prezygapophyses is a deep fossa, 
into the lower margin of which the neural foramen opens. 
The transverse processes in presacrals to 111 and in v are broken — 
and lost. In those vertebre in which they are preserved, these 
processes have been too much distorted to admit of their direction 
being determined with certainty. In the posterior members of the 
series they were evidently much reduced. In Iv a single transverse 
process is preserved which is little more conspicuous than the tuber- 
cular facet. In vi and vir these processes rapidly increase-in length. 
They arise from four roots very much as do those of the posterior 
dorsals in Apatosaurus. They are broad at the base antero-poste- 
riorly, rugose on the anterior margin, and recurved at the distal end. 
The capitular facets for the rib attachment are borne high above 
the centrum throughout the present series. In presacrals 1 and 11 
there is little evidence as to their position, but in 111 a well-devel- 
oped facet is preserved. From this point forward the facets are borne 
by the lateral surface of the prezygapophyses as well as the anterior 
margin of the transverse process, and are supported by a buttress 
descending to the anterior margin of the neural arch. The zyga- 
pophyses, together with the hyposphene-hypantrum articulation con- 
stitute one of the distinguishing features of the vertebrz of this genus. 
Throughout the dorsal series, so far as known, the zygapophyses are 
narrow and placed close to the median line. There is no.evidence of 
dichotomy in the anterior dorsal region as observed in Apatosaurus, 
Camarasaurus, Morosaurus, and Diplodocus. The weakness in the 
zygapophyses is compensated by the strength of the hyposphene- 
hypantrum articulation. The articular surfaces of the former are 
directed horizontally. The mesial surface of the prezygapophysis 
curves rapidly downward and is continuous with the vertical articular 
