400 NATURAL SCIENCE [December 



external divisions must be considered as secondary rings and not as 

 true segments. 



The correctness of this suggestion seems to me to be entirely 

 borne out by the abdominal segmentation of Protocaris in which 

 this secondary ringing has not taken place. If a body-ring were 

 marked round the abdomen of Apus for each of the pairs of limbs, 

 leaving out the most minute at the posterior end (cf. rigs. 1 and 6), 

 we should get a condition, at least for the limb-bearing portion of 

 the abdomen, not unlike that shown in Protocaris, in which a multi- 

 tude of very small segments (though not diminishing quite so clearly 

 as they would in Apus) are in striking contrast with the anal seg- 

 ment. 



It should, however, be noted by the way that this interpretation 

 of the segmentation of Apus which receives such unexpected and 

 welcome support from Protocaris was greatly complicated by the 

 presence in Apus of a varying number of limbless segments in front 

 of the anal segment. These still require explanation ; at present I 

 am inclined to look upon them as secondary reduplications of the 

 anal segment. 



This difficulty must not, however, be thought to stand in the 

 way of my interpretation of the segmentation of Apus. That inter- 

 pretation has already received abundance of support from the fact 

 that the same fixation of rudimentary segments is found in the 

 Trilobites. Inasmuch, however, as the relationship between Apus 

 and the Trilobites is still a matter of discussion, it is especially 

 welcome to obtain direct evidence from a fossil whose close affinity 

 with Apus cannot be for a moment doubted. 



The second point arises from the peculiar shape of the shield. 

 Mr Schuchert describes it as subquadrangular, and quotes Clarke's 

 suggestion that it has probably been subjected to some horizontal 

 distortion in the shale. The longer I contemplate the figure of this 

 shield the more convinced I am that it has simply been flattened 

 out, and that in its original shape it was folded down at the sides 

 of the body. Not only do the two anterior lateral projections of 

 the shield suggest this, but the absence of the usual spikes at the 

 postero-lateral corners of the shield are quite in accord. In Apus 

 these spikes are turned up somewhat on to the back (fig. 6), and in 

 the Trilobites they are spread out wide of the body in the horizontal 

 plane. If the shield were folded down at the sides, these spikes would 

 be a serious danger to the limbs and abdomen, and would be sooner 

 or later dispensed with. Whether, again, the dotted lines running 

 along the shield, shown in Mr Sclmchert's figure (reproduced in fig. 

 1), lend any support to this view, I should not like to say, because 

 we have no means of getting at their true meaning, but they cer- 

 tainly suggest to my mind a dorso-ventral flattening. 





