324 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



upwards and outwards from the median line, describing 

 almost a semi -circle. The upper edge of the semi- 

 circle almost reaches the median line. Thus the inner 

 portion of the dorsal border of each side surrounds an 

 almost closed cavity, which corresponds to the " sternal 

 canal " of the Macrura. 



The Endopleur ite is rectangular in shape, and 

 its length is about twice as great as its width. Four 

 borders may be distinguished. 



The inner border is in contact with the outer border 

 of the endosternite at its upper and lower ends. In the 

 intermediate region it forms the outer boundary of the 

 apodemal foramen. 



The articular border (Ab. -p.) is in contact with the 

 upper part of the arthrodial membrane connecting the 

 coxopodites of the fourth and fifth thoracic appendages. 



The epimeral border (Eb. p.) is in contact with the 

 fourth and fifth thoracic epimera. 



The dorsal border (Db. £>.) bounds the dorsal free end 

 of the endopleurite. 



Above the apodemal foramen the endopleurite 

 becomes fused on its posterior face with the following 

 arthrophragm, and the anterior face of the arthrophragm 

 under discussion becomes fused with the preceding 

 endopleurite. 



All the arthrophragm s of the post-oral cephalo- 

 thoracic region are built on the above plan, that is to 

 say, each somite has one endosternite and one endopleurite 

 at each side. But in some cases the homology is very 

 much disguised. 



The last five thoracic arthrophragms are very similar 

 to the one described, but the anterior arthrojihragms are 

 extremely reduced. It is, therefore, advisable to describe 

 the endophragmal skeleton in two parts, 



