INIA GEOPFEENSIS AND PONTOPOEIA BLAINVILLII. 113 



I will now endeavour to formulavize the distinctive characters of these three primary 

 groups of the Odontoceti, giving them the rank of families. — , 



I. Physeterid^. Costal cartilages not ossified. The hinder ribs losing their tuber- 



cular and retaining their capitular articulation with the vertebrae. The greater 

 number of the cervical vertebrae ankylosed together. Pterygoid bones thick, 

 produced backwards, meeting in the middle line, and not imolutcd to form 

 the outer wall of the postpalatine au'-sinus. Symphysis of mandible of moderate 

 or excessive length. No functional teeth in upper jaw. Mandibular teeth various, 

 often much reduced in number. Lachrymal bones usually large and distinct. 

 Bones of the skull raised so as to form an elevated prominence or crest behind the 

 anterior nares. Orbit of small or moderate size. Pectoral limbs small. Dorsal 

 fin usually present. 



II. Platanistid^. Costal cartilages not ossified. The tubercular and capitular articu- 



lations of the ribs blending together posteriorly. Cervical vertebrae all free. Ptery- 

 goid bones thin, not conforming in their mode of arrangement with either of the 

 other sections. Jaws very long and narrow ; both with numerous teeth having 

 compressed fangs. Symphysis of mandible very long, exceeding half the length of 

 the entire ramus. Orbit very small. Lachrymal bones not distinct from the 

 jugal. Pectoral limbs large. Dorsal fin rudimentary*. 



III. Delphimdje. Costal cartilages firmly ossified. Posterior ribs losing their capi- 

 tular articulation, and only uniting with the transverse processes of the vertebra? 

 by the tubercle. Anterior (2-6) cervical, in most, ankylosed together. Ptery- 

 goid bones short, thui, involuted to form, with a process of the palatine bone, the 

 outer wall of the postpalatine air-sinus. Numerous teeth in both jaws (Monodon 

 excepted), sometimes deciduous. Symphysis of mandible short or moderate, never 

 exceeding one-thii'd the length of the ramus. Bones of the skull not raised into 

 a distinct crest behind the anterior nares. Orbit of moderate size. Lachrymal 

 bone not distinct from the jugal. Pectoral limbs varying much in form and size. 

 Dorsal fin usually present. 



I. The Physeteridw appear to constitute a very natural group f. This may, however, 

 be divided into two weU-marked subfemilies : — 



that Mr. Danvin has informed me that he met with no evidence of the existence of a freshwater Dolphin in the 

 La Plata system of rivers, aud that no mention is made by Azara of any such animal. 



* These characters are subject to modification when more is known of the structure of Ponloporia. 



t Van Beneden insists strongly upon the close affinity of Ph/seter with the Ziphioids : he says, " Comme on le 

 voit, les Cachalots sont pour nous des Ziphioides veritables, portant une rangee de dents fortes et cspacecs sur 

 chaque branche de raaxiUaii'e " (Mem, sur une Kouv. Espece de %!fhius, Mem. do I'Acad. Eoyale de Belgique, 

 t. xvi. 1863). 



b2 



