INIA GEOFFEENSIS AND PONTOPOEIA BLAINVILLII. 95 



marked rugosity of the sux-face of their crowns*, and especially by the broad, rounded 

 lobe, developed on the inner side of the base of the crown of those situated in the pos- 

 terior part of both upper and lower jaws (see Plate XXVI. figs 1, 2 & 3). In the 

 anterior two-thirds they are simple, conical, and slightly incurved. They gi-adually 

 increase in size from the front of the jaws until the fourth from the posterior end of 

 the series, after which they diminish again. Unlike those of most Dolphins, the teeth 

 are implanted by large and generally somewhat twisted and flattened fangs (in the hinder 

 teeth very wide transversely), which fit so tightly into deep alveoli that it is almost im- 

 possible to extract them, even in the dried skull, without injury to the bone. When 

 the mouth is shut they fit closely into the interspaces of the opposite series ; but there 

 is little sign of attrition to be seen anywhere on their sm-face. 



-^ The number of the teeth in the different specimens of Inia examined shows a conside- 

 rable range of variation, presuming that they all belong to one species. In the one now 



described there are gT,^ j^ i^^ =104. The larger specimen in the British Museum from 



OQ,., ,oo 



Ega has qg_^., =109, and also two minute rudimentary teeth in the gum behind the 

 last in the left maxilla. In the smaller skull from the same place there are 



OQ 0(K 



^g_;;^ =110. In the skull in the Paris Museum, brought by d'Orbigny, there are, 

 according to Gervais, 35333=182 ; but in the type specimen in the same museum, taken 

 from Lisbon, the number is given by de BlainviUe as 26^^= 1 O^- Iii the Berlin skull the 



n A nn OQ OQ 



teeth are 33_3^ =131f. Von Martius in his diagnosis of the species gives 291159=114. 



The bones of the hyoid apparatus scarcely differ from those of the ordinary Dol- 

 phins. Their general form is shown in the figures (PI. XXVI. figs. 4 & 5) at half their 

 natural size. The basihyal and thyrohyals are not yet united by continuous ossifica- 

 tion. The stylohyals are thick, subcyhndrical, slightly curved, and somewhat flattened 



towards the ends. 



// 

 Antero-posterior diameter of the basihyal I'O 



Transverse diameter 1'3 



Length of thyrohyal 2-0 



Greatest breadth 0-6 



Distance between the outer extremities of the thyrohyals... 3-4 



Length of stylohyal 2-7 



Greatest thickness 0'4 



The spinal column (PI. XXV. figs. 1 & 2) appears complete to the end of the tail, and 

 . onsists of but 41 vertebrae, the smallest number known in any CetaceanJ. Of these, 



* Some Dolphins of the genus Stem of Gray present a similar though far less marked rugosity ; and indi- 

 cations of it are seen in young specimens of Orea and rseudorca. t Peters, in a letter. 



J As the bones had been separated fi-om each other adncleaned at the time that they came into my hands lor 



VOL. VI. PAET in. ^ 



