16 



The chief modification in the sixth segment of the chest is the development of a 

 third articular surface at the back part of the base of the spine, between the two 

 posterior zygapophyses ; and the somewhat greater production of the ridge which 

 stands out from the fore-part of the base of the spine between the anterior zygapo- 

 physes. The inferior or haemal arch is also augmented by increased length of the 

 ribs. The key-bone of that arch, sterneber or haemal spine, Plate XI. figs. 4-7, 

 repeats the characters of that of the previous segment, save that the sternal articula- 

 tion, s, and the haemapophysial ones, hp, hp, of the central division of the bone are 

 more continuous, as shown in fig. 6. Fig. 5 shows the surface which was presented 

 towards the integument ; fig. 4 that which was turned towards the cavity of the chest ; 

 fig. 7 is a side view showing the four articular cavities for the double condyles of the 

 sixth and seventh haemapophyses ; fig. 6 shows the under surface of this remarkable 

 type of sternal bone. 



In the seventh dorsal vertebra (Plate III. figs. 1, 2, 3), a third articular surface, 

 mz, fig. 1, is developed between the two anterior zygapophyses, z, z, to join that upon 

 the back part of the sixth vertebra; so that there are three zygapophyses, a median 

 and two lateral, on both the fore and the back part (mz 1 , fig. 2) of the arch of this 

 vertebra, making, with the three articular surfaces on each side (fig. 3, c', n', d') for 

 the ribs, and with the anterior and posterior surfaces of the centrum, not fewer than 

 fourteen joints. In this and the two following segments of the back (.Dsandg, 

 Plate I.), the ribs attain their greatest length. In the tenth segment the haem- 

 apophyses cease to articulate below directly with a haemal spine. The median zyg- 

 apophyses continue to be developed both before and behind to the twelfth dorsal 

 vertebra inclusive. In the thirteenth (Plate X. fig. 4) this supplementary articula- 

 tion is suppressed behind ; and the costal articulations have disappeared from the 

 diapophyses d. Those on the neurapophyses are almost circular, «', and those on the 

 upper and posterior angles of the centrum, c', have increased in size. The metapo- 

 physis (m), which was indicated by a protuberance above the diapophysis in the pre- 

 ceding dorsals, begins to assume the form of a rugged thick vertical ridge. The 

 fourteenth dorsal vertebra shows the progressive increase of size of the centrum, and 

 the absence of the median zygapophysis before as well as behind; and in it the 

 costal articulation on the centrum for the penultimate rib is lost, as well as that on 

 the diapophysis for the antepenultimate one, and only the subcircular concave neural 

 surface for the rib remains. In the fifteenth dorsal vertebra the posterior zygapo- 

 physes are convex transversely at their inner border, slightly concave in the rest of 

 their extent; the back part of the neural spine between these processes is deeply 

 grooved ; the metapophysial ridge increases in height and length ; a short and thick 

 anapophysis is developed from the back part of the base of the diapophysis, and on 

 the under part of the anapophysis there is a distinct, nearly flat, articular surface. 

 The sixteenth dorsal vertebra (Plate III. figs. 4 and 5, Plate X. fig. 5) offers a 

 corresponding modification at the fore-part of each neurapophysis, in the develop- 

 ment of a short, strong, wedge-shaped process, p, fig. 5, answering to the parapo- 



