Prof. 0. C. Marsh — A New American Jurassic Dinosaur. 105 



The jaws contain a single row only of teeth in actual use. These 

 are rapidly replaced, as they wear out or are lost, by a series of 

 successional teeth, more numerous than is usual in these reptiles. 

 Figure 6 represents a transverse section through the maxillary, just 

 behind the fourth tooth. The latter is shown in place (1), and below 

 it is a series of five immature teeth (2 to 6), in various stages of 

 development, preparing to take its place. These successional teeth 

 are lodged in a large cavity (<?), which extends through the whole 

 dental portion of the maxillary. The succession is also similar in the 

 premaxillary teeth, and in those of the lower jaws. 



Eig. 7. — Twelftli caudal vertebra of Biplodocus longus, Marsh ; side view, one-sixth 

 natm-al size, c, anterior face for chevron; c\ posterior face for chevron ; s, neural 

 spiue ; z, pre-zygapophysis ; z, post-zygapophysis. 



Fig. 8. — The same vertebra ; view of the underside, size and letters as Fig. 7. 



The Vertehrce. — The vertebral column of Diplodocus, so far as at 

 present known, may be readily distinguished from that of the other 

 8(iuropoda by both the centra and chevrons of the caudals. The 

 former are elongated, and deeply excavated below, as shown in 

 Figures 7 and 8. The chevrons are especially characteristic, and to 

 their peculiar form the generic name Biplodocus refers. They are 

 double, having both anterior and posterior branches, and the typical, 

 forms are represented in Figures 9 and 10, p. 106. 



The Pelvic Girdle. — The most characteristic bone of the two families 

 of Sauropoda previously described is the ischium. In the Atlanta- 



