HAPLODONTIDiE— VERTEBRA OF HAPLODON RUFUS. 573 



about the middle of the series, when, with still increasing length, they also 

 widen again and gradually assume the slight hourglass-like contraction, which 

 is more conspicuous in the lumbar series. The centra all bear costiferous 

 demi-facets, one at each end, to the 9th, which has only one such demi-facet 

 on (he fore end, and none on the other (as well as can be determined in the 

 dried state of the bones examined); the last four (10th to 13th) have full 

 facets upon their fore ends. The spinous processes of the anterior half- 

 dozen dorsals are long, slender, compressed, tapering to a knobbed point, 

 excepting the 1st one, which ends acute, and is abruptly shorter than the 

 next ; the 2d to 6th are subequal and longest ; all these slender acute spines 

 incline strongly backward, and are packed closely in the concavity of this part 

 of the spinal column. On the 7th vertebra, the character of the spinous 

 processes begins to change ; the}' shorten rapidly in vertical length, and at the 

 same time lengthen horizontally, becoming erect and laminar instead of 

 acuminate, thus merging insensibly into the vertically short, horizontally long, 

 rectangular, plate-like spines of the lumbar region. 



The "transverse " processes of the dorso-lumbar vertebrae offer a means 

 of distinguishing between the two series as readily as does the presence of 

 true ribs in one and their absence in the other, whether we pay attention 

 to the homologies of the various elements of which these processes are re- 

 spectively composed, or simply regard their physical appearance. The dorsal 

 diapophysis proper, by which I mean that portion of the complex " trans- 

 verse " process which bears a facet for articulation with the tubercle of a rib, 

 is most projecting on the 1st dorsal, and bears a cupped facet; this process 

 regularly diminishes in length, and in the concavity (soon lost) of its facet, 

 to the 10th vertebra, and is entirely wanting, to all appearance, on the 11th to 

 13th vertebras, with which the corresponding ribs have no connection except 

 by means of the respective centra. From the* upper surfaces of these 

 diapophyses, excepting the 1st one, springs another series of processes, which 

 begins with the 2d dorsal as a simple spur, directed backward, upward, and 

 outward ; these rapidly increase in size with successive vertebvse, and with 

 the 6th acquire a new character, becoming expanded and plate-like, and 

 developing an anterior spur as well as retaining the original posterior one. 

 The formation grows more distinct on succeeding vertebrae to the 10th. A 

 decided excavation of circular shape lies on the under side of this plate ; the 

 anterior corner overlies the pre-zygapophysis of its own vertebra, and the post- 



