a small pari must be ascribed to decrease in size of' (be cranium. 

 It is chiefly caused by the development of a gigantic crista lambdoidea 

 and by the big mastoid process, as well as by the form of both. 

 The crista lambdoidea namely by its size draws the parietal upwards 

 and since in a median direction it extends far to the front, it exerts 

 this influence over a great part of the circumference of the parietal. 

 The strongly developed mastoid process, especially by its frontal 

 position, draws the lower edge of the parietal and the upper edge 

 of the squamosum outwards and therefore lias the same effect at the 

 lower side of the parietal as the crista lambdoidea at the upper and 

 posterior side: the two together produce the concave shape of the parietal. 

 While now the strong development of the crista lambdoidea and the 

 increase in height of the mastoid process also cause an increase in 

 height of the skull as compared with the walrus, it is at the same 

 time broadened by the frontal position of the mastoid processes. For 

 these are placed with the walrus in a slanting forward direction 

 and are also smaller than with Alachtherium. So we cannot wonder 

 that the absolute height and breadth of the skull exceed the corresponding 

 dimensions of the walrus, but that their ratio lies within the limit 

 of variation of recent animals. 



Other differences with Trichechus rosmarus are found at the base 

 of the skull. With the walrus a very large bulla ossea extends from 

 the external edge of the basioccipital and basispbenoid as far as the 

 mastoid process and as far forwards as the fossa glenoidea; a more 

 or less distinct groove separates this bulla ossea, in a rostral and a 

 caudal part. With Alachtherium, however, the bulla ossea is very 

 small and in this respect it deviates distinctly from the Trichechus 

 type. Corresponding to some extent to this circumstance the fossa 

 glenoidea lies far backwards in Alachtherium : the space between 

 the articulation and the mastoid process is very small. While now 

 with the walrus (he fossa glenoidea extends on the jugal process of 

 the squamosum, so that above it the squamosum rises in a slanting- 

 upward direction, with Alachtherium it lies far less free and in front 

 of it the squamosum rises steeply. We shall see later on that this 

 is of importance when dealing with the mandibles to which the 

 cranial fragment was said to correspond by van Beneden. 



A further difference is found at the border between basioccipital 

 and basispbenoid. Not to mention a frontally diverging crista and a 

 roughness on the occipital on both sides behind it, these bones pass 

 gradually into each other in the walrus. In Alachtherium the outside 

 of the sphenoccipital suture is strongly thickened ; a real knob has 

 formed which only at the left side has been preserved. The back of 



