15 



the posterior edge of this groove a slight depression passes 

 outwards at each side parallel to the anterior border. 

 This depression probably marks the boundary between the 

 antennary and mandibular sterna. This groove is better 

 defined on the dorsal side of the epistoma. 



The Labrum (PL III, fig. 20, lab.} is a soft fleshy 

 lobe attached to the middle region of the posterior border 

 of the epistoma. It is surrounded near the middle by a 

 calcareous ring which gives off a median posterior 

 prolongation. At each side of this median plate is a 

 soft fold. 



3. Post-oral Cephalo-thoracic Sterna 

 (PL I, figs. 2, 3, Text fig. 3). 



These are all fused together as a single oval-shaped 

 plate situated between the bases of the paired post-oral 

 cephalothoracic appendages. Transverse grooves are 

 present which mark the division of this region into 

 segments or somites, and which mark the places at which 

 the sterna grow inwards to form the endosternites of the 

 endophragmal system. 



The surface of the fused sterna is concave laterally 

 in order to accommodate the abdomen, which is always in 

 a flexed condition. This concavity is especially well 

 marked in the males. The surface of the sterna is, how- 

 ever, convex antero-posteriorly. 



On the sternum of fifth thoracic somite are two small 

 tubercles (P.) which fit into two concavities on the 

 abdomen and thus form an effective locking apparatus 

 which keeps the abdomen in position. These are 

 especially large in the males. 



The sternum of the sixth thoracic somite of the 

 female bears a pair of large openings which are the 

 external genital openings. 



