402 NATURAL SCIENCE [December 
We note, particularly at the outset, that Mr Schuchert evidently 
assumes that the head of Dipeltis 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,* 
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 comes 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 Apus-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-like 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—ze. 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 fee tal repetition of that heat 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 Q. Journ. Geol. Soc., vols. li. and lii. 
