SINUS PARANASALES EARLY TERTIARY MAMMALS 143 



(Sisson, '14, fig. 212), although the posterior superior portion 

 of the cavities has been lost in the fossil form. Compared to 

 the modern dog, these cavities are smaller, measuring in the 

 present specimen fifteen millimeters in length by seven in breadth. 

 The cavities are, however, more numerous and more elongate. 

 There are three of them to each side of the median septum, the 

 medial one of which is the larger and longer, while that on the 

 lateral surface is about half the size. Thin plates of bone sepa- 

 rated these cavities. There are no evidences of the blood sup- 

 ply, since the cast had been somewhat eroded before collection. 



The sinus maxillaris is not completely preserved, since the 

 portion of the cast which projected into the maxilla is lost. 

 The base of this projection, which lies well forward, is present 

 and it indicates a more extensively developed sinus than occurs 

 in modern dogs. There are no superior divisions of this sinus. 

 The impressions of the fluting of the ethmoturbinals are pre- 

 served very clearly. These impressions indicate a cavum nasi 

 of larger capacity than the modern dog possesses, and likewise 

 apparently would indicate a much keener sense of smell, although 

 the cavum nasi is greatly narrowed at the point of entrance of 

 the olfactory nerves. 



The brain cast is very well preserved, especially the cerebellar 

 portion. The form of the brain is essentially dog-like, though 

 not so richly convoluted as in the modern dog. The cat-like 

 characters, evident in its skeletal make-up, have made little 

 impress on the form of the brain. Its form is quite distinct 

 from that of the cats of the same period. 



A study of these two casts leads us to believe that the origin 

 of the sinus paranasales is to be found, not in the early mammals, 

 but in their ancestors, and probably their remote ancestors. 

 Just how far back we would have to go to get the phylogenetic 

 beginning of these interesting cavities is uncertain. Their na- 

 ture in the early reptiles and amphibians is unknown. Descrip- 

 tive paleontology has, so far, paid no attention to these inter- 

 esting phases of the morphology of extinct forms, and it is cer- 

 tainly well worth developing. It will not only give new life 

 to the science itself, but will assist in the interpretation of many 

 features of vertebrate morphology which are now obscure. 



