EVOLUTION 33 



parts. The palatal processes of the maxillaries and the palatines 

 have been deepened vertically ; the median sutures of these 

 bones, open throughout life in primitive IMuridse, have closed and 

 disappeared ; the ectopterygoid plate of the alisphenoid supporting 

 the hinder end of the maxilla has been greatly developed, and the 

 presphenoid and the fore-part of the basisphenoid, owing to the 

 intrusion of the alveolar capsules into the sphenorbital fissure and 

 the consequent driving inwards of the first and second divisions 

 of the great trigeminal nerve, have become laterally compressed 

 and vertically deepened. 



Under the influence of the deep part of the masseter lateralis 

 muscle the zygomatic process of the maxilla, particularly its lower 

 root which forms the outer wall of the infraorbital canal, has 

 become unusually strong and characteristically formed. The 

 temporal muscle, and particularly its anterior portion, is greatly 

 increased in size in all Microtina? ; it has made room for itself in 

 the orbito-temporal fossa by driving the zygomatic arch outwards, 

 by compressing laterally the interorbital region which in the 

 highest forms becomes extremely narrow, and by driving the eye 

 forwards. The latter organ, becoming less and less important as 

 Microtinse become more strictly fossorial, is reduced in size and 

 displaced, being lifted up by the molar capsules as well as pushed 

 forwards by the temporal muscle. The squamosal bone, which 

 gives origin to the greater part of the temporal muscle, has like 

 this muscle risen in importance ; it shows a well-marked tendency 

 to encroach upon the frontals anteriorly and upon the parietals 

 and interparietal behind in many Microtine genera, and on the 

 shoulder of the braincase it forms a more or less well-marked 

 post-orbital process or crest for the origin of the tendinous portion 

 of the temporalis. Partly in consequence of the increased develop- 

 ment of the ectopterygoid plate, but partly owing also to increased 

 size of the pterygoid internus muscle, the pterygoid fossa has been 

 deepened on each side. In the mandible the muscular specializa- 

 tions are reflected in the form of the coronoid and angular processes 

 and in the strength of the crista masseterica ; where the inser- 

 tions of the pterygoid internus and masseter lateralis muscles have 

 become tendinous and concentrated, the angular process has 

 suffered reduction. 



Fossorial specialization has led to the reduction of the eyes and 

 external ears, shortening of the limbs and tail, broadening of the 

 head and thickening of the body, and to many modifications of 

 the hands, feet, and fur. In the skull it is betrayed by the 

 flattening of the dorsal surface and, in extreme types, by the pro- 

 jecting and straightened (proodont) incisors, shortened nasals, 

 shallowed rostrum, and forwardly inclined occiput. The auditory 

 bulla3 become specially developed for underground life ; the 

 cavities of the middle ear, ma.stoid portion, and the swollen 

 tegmen tympani are filled, in the most modified forms, with 

 spongy tissue, and the canaliculus tympanicus becomes completely 



V.L. D 



