41 



(1) There is no median plate. This only begins at 

 the level of the fourth thoracic arthrophragm. 



(2) The post-oral sterna anterior to the fourth 

 thoracic arthrophragm are all fused together. Con- 

 sequently there can be no broad plate-like endosternites 

 formed as in growths between the somites. The small 

 endosternite present in each somite of this region is rod- 

 like, and represents merely the articular border of the 

 typical endosternite. (The third thoracic endosternite 

 has the form of a fairly broad and deep plate, and is 

 therefore an exception to this rule.) 



(3) The epimera in front of the second thoracic 

 arthrophragm are fused together, so that the endopleurite 

 in these somites are extremely reduced and represent only 

 the articular border of the typical endopleurite. 



Here, as in the posterior thoracic region, each 

 endopleurite gives off a posterior out-growth, which fuses 

 with the following arthrophragm. So that pleural 

 muscle chambers and sternal muscle chambers may be 

 made out, but owing to the rod-like nature of their con- 

 stituent parts, they have a very different appearance from 

 the muscle chambers of the posterior somites of the 

 thorax (see Text fig. 6, also PI. Ill, fig. 18). 



The third thoracic arthrophragm (Text fig. 7, E.) 

 arises between the third and fourth thoracic somites. 



Each endosternite (PL III, fig. 18, e.st.S) differs from 

 that of the typical arthrophragm described above. It 

 arises merely from the oviter edge of the sternum, so that 

 the two endosternites are separated from each other by 

 the entire width of the sternum in this region. The 

 endosternite is a broad and deep plate facing downwards 

 and backwards. The dorsal and inner corner is prolonged 

 inwards and backwards and almost meets the similar part 

 from the other side. 



