168 MAMMALIAN DESCENT. [Lscr. VII. 



the Shrews in having no cheek bones. When the 

 development of these long-snouted types is studied, we 

 find that the main skull beam in front of the pituitary 

 body and its shallow cup, the sella turcica, is mainly 

 composed of one large solid bar of cartilage just as 

 in such diverse types, at each end of the Vertebrata, 

 as the Skate and the Whale. Up to the olfactory 

 region (middle ethmoidal territory) this single solid bar is 

 flanked by a shorter bar, right and left, and these two 

 bars are a direct continuation of the cartilage, which 

 invests the cranial notochord the " investing mass " of 

 Eathke (the continuation in the head of the axis of the 

 creature). The shorter paired cartilages were called by 

 him the trabeculse cranii or rafters of the skull ; the middle 

 projecting bar I have simply called the intertrabecula ; 

 Rathke's term has become classical. In the Skate the 

 roofs of the nasal organs lie back against the front of the 

 skull, but in this, as in most Mammals, the whole nasal roof 

 is continued to the end of the snout, and is folded over to 

 form the alae-nasi, the cartilages that encircle the outer 

 nostrils. Thus the long rostral median cartilage does not 

 project as a free rod, as in the Skate, but its crested upper 

 edge is everywhere confluent with the right and left carti- 

 laginous roof-plates of the labyrinth of the nose. There- 

 fore, from the outer nostrils to the naso-palatine openings, 

 over the soft palate, there are two tubular passages ; over 

 these lie the turbinals, or coiled bones, on which, in the 

 region nearest the skull, the olfactory nerves or nerves of 



