258 THE APODID^E PART ii 



dorsal shell of an Apus-like animal, we have been 

 mainly influenced by the following considerations, 

 which must be admitted to be of some morphological 

 importance. 



(i) The position of the head in the shell seems to 

 point decidedly to such an origin. If the shell had 

 been formed by the bending down of the sides of a 

 dorsal fold, the head would either project anteriorly 

 as in the Cladocera, or, if it came between the shells 

 at all, could only do so by itself bending round ven- 



FIG. 58. Cypris fasciata (from Bronn's Klassen und Ordnungen des Thierrdchs) 

 to show the position of the head in the shell for comparison with the following 

 figures and with Fig. 57. 



trally,as shown in different stages in Figs. 60, 61, and 

 62, or by the growing forward of the halves of the shell 

 so as to cover the head ; this latter method is, for many 

 reasons, not a very probable one. In the Ostracoda 

 we find the " face " deep back in the shell, pointing- 

 forwards in a way difficult to explain on any other 

 hypothesis than that which we put forward. These 

 projecting parts of the shell are the lateral halves of the 

 shovel-shaped ridge which projected so far forwards 

 in the original Trilobite ancestor of the group. If we 



