402 NATURAL SCIENCE [December 



We note, particularly at the outset, that Mr Schuchert evidently 

 assumes that the head of Dvpeltis corresponds to the head of Apus 

 with its five segments. There is no reason to doubt the correctness 

 of this assumption. 



When analysing the differences between Apus and the Trilobites, 1 

 I came to the conclusion that the essential distinction between them 

 was as follows : — In Apus, after the formation and fixation of the 

 head region out of five fused segments, the cephalic shield grew 

 backwards as a large free carapace over the trunk segments, which 

 remained cylindrical, whereas in the Trilobites the head shield did 

 not grow backwards freely over the body, but was repeated, by 

 the familiar process of segmental repetition, on each of the trunk 

 segments, giving rise on them to their pleural extensions. This 

 curious difference in the mechanism of development resulted in two 

 such apparently distinct forms as Apus and the Trilobites. 



Dipeltis now conies on the scene. Its general appearance is 

 that of Apus, but its repetition of the head shield is that of the 

 Trilobite. The head shield and its two large segmental repetitions 

 together appear as if they imitated the true free carapace of Apus. 

 They only appear to cover the trunk segments, as the carapace of 

 Apus covers its cylindrical trunk. Further, the trunk segments 

 also show in their small pleurae a slight repetition of the head 

 shield, or rather of the last of the two large repetitions of the head 

 shield. These are unmistakable Trilobitic characters. On the 

 other hand, the anal segment with its pair of cercopoda, the arrange- 

 ment of the large eyes with anterior eyes or pores at the front of 

 the head, the smooth round forehead without glabella or furrows, 

 are unmistakably apodidan features. The animal looks remarkably 

 like a cross between an Apus and a Trilobite ! The only explana- 

 tion I can suggest of this singular creature is that it resulted from 

 a second attempt on the part of the main- or ^Ms-stem of the 

 crustacean phylum to adopt the Trilobitic modification ; that is, for a 

 second time these primitive Crustacea tried the segmental repetition 

 of the early head shield instead of its free backward extension as a 

 carapace. I would explain the Apus-Uke appearance of this second 

 attempt as being due to the fact that it started from a true Apus, 

 whereas the Trilobites owe their peculiar characters to the fact that 

 they branched off before the Apus type was fixed — i.e. while the 

 head region was still in process of formation — hence the glabella, 

 the transverse furrows, and the uncertain position of the eyes on the 

 head shield, with the three longitudinal regions running along the 

 dorsal surface due to the segmental repetition of that head shield. 



Turning, in conclusion, to the eyes in figure 3, these are shown 

 very like those of Apus as to position and arrangement, while in front 



1 V- Jowrn. deal. Soc, vols. li. and lii. 



