132 



Mr. W. K. Parker. 



[Jan. 29, 



i. The angular part of the lower jaw is greatly incurved forming a 

 remarkable hollow inside. 



In the Endo-cranium there are some very curious structures that 

 differ from what we find in the high forms of Mammalia, but which 

 mostly agree with what is seen in the Sauropsida. 



a. The nostrils are sub-terminal, and give off large tongue-shaped 

 cartilages to protect Jacobson's organs. 



b. The whole nasal labyrinth is small, especially in the young, not 

 more than half as large as in an average placental Mammal, and the 

 cribriform plate is less depressed in front, very limited in size, and is 

 square in form. 



c. The orbitosphenoids do not form the presphenoid by meeting 

 together below, but the presphenoid is as independent as the basi- 

 sphenoid. 



d. There is no special optic foramen in the orbitosphenoid, but the 

 optic nerve passes through the common sphenoidal fissure with the 

 orbital nerves and the first branch of the fifth ; the second branch , 

 like the third, has its own foramen rotundum, as in Man, and many 

 other Eutheria. 



e. The next character is one of the most important ; it is this, 

 namely, that the orbitosphenoids are flush with the alisphenoids. 

 The latter, which are extremely large, ossify a tract of the general 

 cartilaginous side-wall of the embryonic skull — the highly developed 

 chondro- cranium — and not a free flap of cartilage, merely continuous 

 with the basal bar, as in the Eutheria. For in these latter the more 

 "bulky brain pushes out the lower part of the side-wall of the skull, 

 leaving for some time a band of cartilage, which runs free from the 

 alisphenoid, passing from the orbitosphenoid up to the super- occipital. 

 In front, the orbitosphenoid is confluent with the ethmoid, so that but 

 for the breach in the wall made by the alisphenoid, there would be, 

 even in placental Mammals, a chondro-cranium very similar to that 

 of the Skate. This breach does not take place anywhere among the 

 vertebrate types until we get above the Marsupials. The other cha- 

 racter just mentioned, namely, the absence of a special optic foramen, 

 is of similar import; there is no such a foramen in the " Amniota" 

 until we are among the placental Mammalia. 



f. The alisphenoid helps to form the drum cavity by developing 

 behind its small external pterygoid process, a shell-like growth, simi- 

 lar to the " anterior tympanic recess " of Carinate birds. Thus, as 

 the squamosal is a labyrinth of air cavities opening into the upper 

 part of the drum cavity, these and the tympanic recess in the ali- 

 sphenoid greatly enlarge the space for air. Indeed, not only those 

 parts, but the mastoid region of the auditory capsule, and the sides 

 and top of the occipital arch, all become pneumatic, as in Crocodiles 

 and Birds. 



