, SKULL OF VERTEBRATA. 465 



inwardly projecting processes. Below it, lie tlie skeletal parts of 

 the maxillo-palatine apparatus, and to these a median lamella of 

 cartilage, which forms the wall of partition of the nasal cavity, is 

 sent down. The vomer is developed as an investing bone on this 

 plate (Fig. 253, Vo). Two ethmoidal pieces are formed by the 

 ossification of the two lateral halves of the ethmoidal cartilage, and 

 of the lamellar processes (turbinate bones), which are given off from 

 it. They bound a portion of the cranial cavity in front of the pre- 

 sphenoid, and are fenestrated to allow for the passage of the olfactory 

 nerve. In Ornithorhynchus there are only two orifices in this region, 

 but they are many more in all other forms, so that this portion is 

 converted into the cribriform plate. An unpaired bone is formed by 

 the fusion of the two lateral halves with the median piece (lamina 

 perpendicularis) (Fig. 253, ffih). 



The turbinals vary greatly in character, and by the development 

 of multi-ramified lamella? aid in increasing the size of the nasal 

 cavity. The ethmoidal segment is, as a rule, covered by other bones, 

 and especially by those which form the maxillo-palatine apparatus ; 

 and that to such an extent that no part of it at all can be seen 

 from the outside. Except in some Edentata, it is in the Primates 

 only that a portion of the lateral surface reaches to the median 

 boundary of the orbit, where it forms the " lamina papyracea." 



The lachrymals and the nasals form the investing bones of the 

 outer surface of the ethmoidal region. The former (L) are less 

 constant, and often seem to pass into the neighbouring bones, so 

 that they, as for example in the Pinnipedia, are absent as separate 

 pieces. They are wanting, also, in the Delphinoidea. As in Reptiles 

 and Birds they form part of the anterior wall of the orbit, and also 

 appear on the facial surface of the skull, from which they extend 

 backwards to the median wall of the orbit, in the Primates. 



As to the nasals (No), they exhibit merely subordinate variations, 

 which are expressed partly by degeneration (Cetacea), partly by a 

 considerable increase in size. Their size is proportional to that of 

 the nasal cavity, and is correlated with an elongation of the facial 

 portion of the skull. They are small in the Primates. 



§ 351. 



The most important peculiarities in the Mammalian skull are 

 seen in the part which is developed out of the primitive branchial 

 skeleton. A bone which corresponds to the quadrate lies on the 

 outer surface of the auditory capsule. It forms an auditory ossicle, 

 the incus. 



The skeletal parts, which are developed in front of the quadrate, 

 and along the base of the skull, are intimately connected with the 

 cranium. 



The pterygoids (Fig. 253, Pt) are generally flat pieces of bone, 

 which are placed on the inner surface of the large processes which 



2 H 



