41 



the Halicore Tabermculi, 7 cervical, 19 dorsal, 3 lumbar, 3 pelvic, 

 and 27 caudal vertebra: ; in all 59 vertebra. J found, as he also 

 describes, that the first four pairs of ribs reached the sternum, through 

 the medium of cartilages ; all the others terminated freely in the mass 

 of abdominal muscles : the 10th to the 15th are the longest, the last 

 is the shortest. The affinity of the Dugong to the Pachydermata is 

 thus again illustrated by the great number of the ribs. The lower 

 jaw is articulated to the cranium by a true synovial capsule, reflected 

 over cartilaginous surfaces, and not, as in the carnivorous Cetacea, 

 by a coarse and oily ligamentous substance. 



Dentition. 



" My attention was particularly directed to the state of the denti- 

 tion in the Dugongs of different sexes, which I have thus had the 

 good fortune to examine ; from which it would appear that, as in 

 the Narwhal, the permanent tusks of the female are arrested in their 

 growth, and remain throughout life concealed within the substance 

 of the intermaxillary bones and the alveolar integument. The ca- 

 vity of the tusk is in like manner filled up by the secretion of the 

 pulp which retrogrades in the course of its absorption, and hence 

 the tusks are solid, like the corresponding tusks in the female Nar- 

 whal, or at least present only a shallow cavity at their expanded 

 and distorted base. The form of the tusk from this part is irregu- 

 larly cylindrical, and it diminishes to an obtuse point at the opposite 

 or lower extremity, which is perceptible only in the dry skuU. 



" It is remarkable that in all cases the external parietes of the aU 

 veolus of the abortive tusk is wanting opposite its base, and this 

 occurs even in the young female Dugong, when the base of the per- 

 manent tusk is near the lower extremity of the deflected portion of 

 the intermaxillary bone ; but as the pulp and the base of the tooth 

 ascend, (or rather appear to ascend, in consequence of the elonga- 

 tion of the bone and the teeth,) the vacuity also ascends, and is situ- 

 ated in the adult at the upper part of the external surface of the de- 

 flected portion of the intermaxillary bone*. In the male the per- 

 manent tusks project beyond the jaws, and manifest, by the deep 

 conical cavity at their base, the persistence of the formative pulp 

 and their continual growth and renovation. These tusks also differ 

 from those of the female, in not being expanded at their bases, but 

 continuing of uniform diameter from one end to the other ; the pro- 

 jecting extremities of the tusks are bevelled off from within, out- 

 wards and downwards, and terminate in a sharp chisel-edge. Only 

 a very small portion of the tusk projects from the jaw, (in which 

 circumstance the Narwhal differs most widely from the Dugong,) at 

 least seven- eighths of the tusk are imbedded in its socket, and the 

 socket is entire throughout its whole extent, the exterior of the In- 

 termaxillary bones generally presenting an unbroken surface, which, 



* The skull of the female Dugong figured by Ruppell {loc. cit.) exhibits 

 this characteristic vacuity in the parietex of the socket of the tusk. The 

 contained teeth were cylindrical and conical. 



