26 THE VOYAGE OF H.M.S. CHALLENGER. 



ticuous sides of the body of the seventh and eighth dorsal vertebrae, but not to any 

 transverse process. In Mesoplodon soiverhyi ^ the anterior seven ril)3 were articulated hj 

 both heads and tubercles to their approjjriate vertebrae, whilst the eighth, ninth, and 

 tenth ribs had each only a single articulation with the transverse process from the side 

 of the body of the corresponding dorsal vertebra. In Mesojolodon austraUs, again, the 

 anterior six ribs were articulated by head and tubercle to body and transverse process ; the 

 eighth and ninth only to the transverse process from the side of the body of the corres- 

 ponding vertebrae ; whilst the seventh "left rib was attached by the former method and 

 the seventh right by the latter only.^ In my specimen of Mesojjlodon layardi, as in Meso- 

 plodon australis, the seventh dorsal vertebra with its pair of ribs was the vertebra of 

 transition. 



Sternum.- — The sternum consisted of three bony segments articulated together by 

 intermediate bands of cartilage. A band of unossified cartilage about 1 inch deep was 

 attached to the anterior border of the manubrial segment. Between the manubrial and 

 second segment was a mesial foramen about 1^ inch long and fths of an inch wide, and a 

 similar hole was between the second and third segments. The manubrium was 5 inches 

 long by 4| inches in its greatest transverse diameter ; its inferior surface had a faint 

 mesial ridge, its superior surface was concave. Its inferior border had a mesial notch. 

 The second segment was 3| inches long and notched both at its anterior and posterior 

 liorders, where it contributed to form the boundaries of the mesial sternal foramina. The. 

 third segment was 2^ inches long and notched only at its upper border. The sternum 

 articulated with four pairs of ribs. The first with the cartilaginous band at the anterior 

 border of the first segment-; the second with the plate of cartilage between the first and 

 second segments ; the third with the corresponding band between the second and third 

 segments ; the fourth with the sides of the posterior border of the third segment (PI. I. 

 fig. 4). 



In von Haast's specimen the sternum consisted of four segments, of which the fourth 

 was divided into a right and left portion. It also articulated with five pairs of ribs. In 

 Mesoplodon australis four segments articulating with five pairs of ribs were also present, 

 and in the immature sternum of Mesoplodon grayi four segments were recognised by 

 Professor Flower. In Mesoplodon sowerbyi five segments are figured by MM. Van 

 Beneden and Gervais,^ the last being divided into two lateral halves. It is very probable, 

 that in the immature skeleton of Mesoplodon layardi I am now describing, the fourth 

 segment had not been ossified. 



Hyoid hone. — The only representatives of the hyoid apparatus consisted of the pair of 

 stylo-hyals, each of which was broken, but the articular surface apparently at the cranial 

 end was preserved. 



' Van Beneden and Gervais, Osteograpliie des C^taces, pi. xxii. fig. 1. 



2 See Flower in Trans. Zool. Soc, 1878, p. 431. ^ Osteograpliie des Cetaces, pi. x.\ii. fig. 2. 



