10 H. B. POLLARD. 



the nerves being almost in an embryonic condition. Apparently in 

 all the S. American Silnroids ossification begins at a remarkably 

 early stage. 



The auditory region shows much replacement of cartilage by bone,, 

 only irregular and asymmetrical projections being left. Cartilage only 

 remains in fact in the neighbourhood of the hyomandibnlar articulation. 

 A pterotic ridge is little developed and the hyomandibular articulation, 

 which is very great in extent anteroposterior^, may be said to be on 

 the pterotic ridge itself. The cartilaginous floor of the brain case is 

 only represented by three isolated blocks, one unpaired and median 

 at the level of the auditory labyrinth, and a pair situated below the 

 foramen of exit of the Facial and Trigeminus nerves. 



The supraorbital band is the only remaining cartilage of the 

 posterior part of the orbital wall. It is somewhat triangular in section. 

 At the middle of the orbital region the epiphysial bar (Eph.) passes 

 across the supracranial fontanelle. In the anterior border of the 

 epiphysial bar at the median point is a notch where the epiphysis 

 rests. 



In front of the level of the epiphysial bar a portion of the carti- 

 laginous side wall is left and shown foreshortened in the figure (Fig- 5) 

 but there is no antorbital process and no bridge above from the 

 orbital to the rostral region (in other words the olfactory lobes and 

 nerves are not roofed over by cartilage). The internasal septum takes 

 the form of a distinct rostrum which however does not project beyond 

 the level of the anterior narial opening. 



In the cranial floor behind the rostrum is a pair of small fonta- 

 nelles in the cartilage. One is indicated by the shading on the right 

 of the drawing. These lodge minute projections from the base of the 

 forebrain. 



Behind the epiphysial bar the skull cavity widens out very remark- 

 ably. From the epiphysial bar forwards it is much smaller as shown 

 in the figure. The hyomandibular (H.M.) articulates with the skull 

 by an enormously long articulation and only along the articulation 

 does much cartilage remain. There is, however, a strong process 

 downwards and backwards which supports the operculum without the 

 intermediation of an opercular cartilage. 



Another block of cartilage remains in the symplectic and quadrate 

 region, the intervening portion having been replaced by perichondrial 

 bone. 



