524 CETACEA. 



form posterior half. In adults the maxilla developes a thin plate below the origin 

 of the zygomatic portion of the malar, and the orbital process of the maxilla so ex- 

 pands over it that the bone is firmly locked to the skull ; while in younger skulls 

 the malar is easily removed and indeed falls out. The round anterior extremity 

 projects considerably downwards, defining a deep infra-orbital notch. The head 

 of the bone anteriorly is also traversed by a longitudinal furrow from the lower 

 margin of the orbit. The zygomatic portion in the adult is occasionally supported 

 by a thin sheath derived from the frontal. 



Maxilla (PI. XXXIX, figs. 1 and 2, and PI. XL, figs. 17 and 18). —The 

 dental portion is a long laterally compressed rod, tapering towards its free extrem- 

 ity, and from the external and superior surfaces of its extremity springs an 

 upwardly and backwardly projecting portion which overlaps the frontal between its 

 naso-orbital processes, and which bears the crests which are so distinctive of this 

 dolphin. 



The external surface of the maxilla, behind the open end of the dental furrow, 

 presents, in young skulls, two short parallel upwardly curved ridges; but these 

 are confluent in adult skulls and continuous at their upper extremities with the 

 margins or walls of an upwardly curved fossa which terminates immediately below 

 the orbit. In young skulls, the fossa communicates by an anterior round slit, 

 and posterior to this by a long narrow fissure, with the backward continuation of the 

 dental cavity which forms the outer wall of the backward continuation of the true 

 maxillary fossa or antrum ; but it is marked by imperfections of ossification and by 

 a permanent fissure which leads from the antrum to the fossa. The antrum has 

 its margins usually connected by long spicules or bands, opening posteriorly by 

 two orifices which are formed by the backwardly projecting process of the inner 

 wall of the fossa. This latter is applied to the palatine and forms the lower 

 border of the infra-orbital canal, into which the superior orifice leads directly, 

 while the inferior opens posteriorly by an arched margin along the lower border 

 of the inner plate of the maxilla (defining the outer wall of the fossa) and leads 

 into the pterygoid sinus. The inner wall of this orifice is formed by the outer 

 surface of the external plate of the maxilla in the fossa and by a portion of the 

 palatal. Above the external imperfection of ossification, three to four small oval 

 foramina open from the infra-orbital canal, and doubtless transmit branches of the 

 infra-orbital vessels and nerves. The inner plate of the maxilla, from what has been 

 said, thus apparently forms the outer wall of the fossa, but if the upward twisting 

 of the bone is kept in view, and also its relations to the pterygoid and palatal bones, 

 this portion may be regarded as the true palatine surface of the maxilla, as it sup- 

 ports the lateral, muscular extensors at this part of the palate. Such being the case, 

 the palatal bone is not so abnormally situated as at first sight one would be led to 

 suppose. When the palatine is removed from the posterior extremity of the maxilla, 

 the commencement of the infra-orbital canal is enlarged, as that bone forms its pos- 

 terior boundary defining it as a circular orifice, the floor of which is formed by the 

 palatine ridge-like process of the maxilla, and to the under margin of which the 



