166 O. C. Marsh—New Family of Dinosaurs. 
The former are elongated, and deeply excavated below, as 
shown in Plate IV, figures 4 and 5. The chevrons are espe- 
cially characteristic, and to their peculiar form the generic 
name Diplodocus refers. They are double, having both anterior 
and posterior branches, and the typical forms are represented 
in figures 6 and 7 of the same plate. 
THE PreLvic GIRDLE. 
The most characteristic bone of the two families of Sawropoda — 
previously described is the ischium. In the Aélantosauride, 
the ischia are massive, and directed downward, with their ex- 
panded extremities meeting on the median line. In the Moro- 
sauride, the ischia are slender, with the shaft twisted about 90°, 
directed backward, and the sides meeting on the median line, 
thus approaching this part in the more specialized Dinosaurs. 
The ischia referred to the genus Diplodocus, representing the 
new family here established, are intermediate in form and post- 
tion between those above mentioned. The shaft is not 
expanded distally, nor twisted, and was directed downward 
and backward, with the ends meeting on the median line. 
SIZE AND HaBits. 
The type specimen of Diplodocus, to which the skull here 
figured apparently belongs, indicates an animal intermediate 
in size between Atlantosaurus and Morosaurus, probably 40 oF 
# 
maxillary bone contains eight teeth, and at the premaxillary = 
suture measures 26™" in thickness. The series of teeth occupy 
a space of 70™. A second specimen of apparently the sam 
species has since been found in Wyoming. 
The geological horizon of all the Sauropoda from the Rocky 2 
Mountain region is in’ the Atlantosaurus beds of the uppe 3 
Jurassic. No Cretaceous forms of this group are known. 
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