60 THE VOYAGE OF H.M.S. CHALLENGER. 



process parallel to that on the tergum ; these spines are longer in the successive 

 segments. 



The posterior region of the thorax differs from the anterior in the direction of its 

 .segments as well as in their form. 



Each of the three segments which are comprised in this region of the thorax, 

 are considerably stouter than any of the anterior ones ; their margins, instead of 

 being concave forwards, are convex forwards and concave backwards ; each segment, 

 in fact, appears as if composed of two approximately straight pieces meeting in the 

 middle line at any obtuse angle ; in the last of the three segments the posterior 

 margin is less curved than in either of the preceding segments; this segment conns 

 therefore to be triangular in shape and considerably longer antero-posteriorly than the 

 rest. 



The abrupt transition between the fourth and fifth segments of the thorax, which 

 can be understood by an inspection of fig. 1 of PI. XL, implies a comparatively 

 limited surface of articulation between the two, and, as a natural consequence, the 

 slightest rough handling causes the body to break at this point. 



The transverse diameter of the three posterior thoracic segments decreases pro- 

 gressively from before backwards, so that the third is the narrowest ; each segment is 

 traversed by a median furrow which divides it into right and left halves ; the anterior 

 boundary line of the segment dips in slightly at the commencement of this furrow, which 

 is deeper in the first than in either of the two succeeding segments ; the rest of the 

 segment in every case is convex, with the exception of a narrow anterior and posterior 

 boundary line which is flattened and somewhat upturned ; the anterior flattened margin 

 is produced at the lateral extremity of the segment into a forwardly directed spiny 

 process like those of the anterior thoracic segments, but longer ; these processes are 

 subequal on all the three segments ; behind this process the lateral margin of each 

 segment is produced into a minute triangular process at the base of each of the larger 

 spines ; between this process and the hinder margin of the segment is situated the 

 epimeron, which is extremely small and decreases in size from before backwards. 



The abdominal segments, as in all other species, are fused together to form a 

 shield, at the termination of which the anus opens, so that there is no telson. 



The abdominal shield is roughly triangular in outline with convex margins ; the lateral 

 margins are produced forwards beyond their junction with the anterior margin into a 

 short spinous process, which corresponds with those of the thoracic segments. The 

 dorsal surface is divided into three areas — two lateral somewhat reniform convexities 

 which commence at the anterior but do not reach the posterior margin of the shield ; 

 in the " hilum " of each of these convexities is a minute oval convexity. The surface 

 of these lateral convexities is punctulated. 



The middle of the abdominal shield is occupied by a narrow area which is T-shaped, 



