510 F. G. SINCLAIR. 
It is not easy to account for these facts. Newport (3), im 
his monograph on the Chilopoda, 1844, held that the dorsal 
arch in the Chilopoda is composed of four pieces of which 
the two median ones are the last to be united, these two 
median pieces form the scutum and the two lateral ones the 
episcutum. Bode (1), in 1877, said that the body-ring of 
Polyxenus belongs to the pentazonal type, and describes 
the dorsal portion of the zonite as consisting of two large 
pleure and a relatively small tergum. 
It would be natural to suppose that in the case of Platy- 
desmus, the conditions of Polyxenus had been carried a 
little further, and that the tergum had been reduced to the 
thin median intercatated plate. It is possible that this may 
be the case. On the other hand it is possible that the inter- 
calated plate may be a completely new structure, and that the 
two side plates may represent a single tergal plate that has 
been divided and jointed in the middle longitudinal line. 
The difficulty in the way of accepting the first of these ex- 
planations, is that the keels which should be a prolongation 
of the sides of the terga, judging by the analogy of the keels 
of Polydesmide, would in this case be an extension of the 
pleure. I thought at first that the foramina repugnatoria 
might throw some light on the subject. Humbert and 
Saussure describe them as beginning in the fifth segment. 
Careful dissection and examination by means of sections failed 
to show me any trace of these glands, and I thought that as 
these glands are typically organs of the terga their absence 
was evidence that the two side divisions were not terga but 
pleura. ‘his, however, is contradicted by Craspedosoma, 
which shows, as | have already said, the same formation of 
the zonite but has well-marked repugnatorial glands and 
foramina. Of course, it is not impossible that the keels and 
repugnatorial glands should have been formed in the pleure 
instead of in the terga, still their occurrence is a difficulty to 
my mind. 
The second possibility that I have mentioned would seem 
the most probable to me if there were anything in the way of 
