42 DR. G. C. BOURNE ON THE EANINID.E : 
he distinguished in it. Hence there is nothing to correspond with the selhi 
turcica anterior of the Astacnra. In Droinia vulgaris much the same elements 
can be identified as in the Raninidre, but the sternal and pleural apodonies 
of intersegment ix/x do not meet to form a sternal ciinal. Similarly the 
sternal and pleural apodemes of intersegment viii/ix unite on each side to 
form a curved shelf running inwards from the thoracic wall, but leave a wide 
space open in the middle line. The pleural apodemes vi/vii are fairly well 
developed and project some way inwiirds and backwards, but are far from 
approximating in the middle line. The intersegment v/vi is fairly stout, bnt 
is simpl}' an arthrophragm without any ingrowths that can be distinguished 
as endosternites, and though the sixth pleura are well developed, they have 
no inwardly projecting apodemes separating them from the membrane that 
corresponds with the fifth pleuron. In all these respects Dromia has 
departed further from the Astacuran condition than have the Eaninidas. In 
another matter also, the Eaninidas stand closer to the Astacura. In the last 
named, for example in Nephrops, the ninth, eighth, and seventh sterna do not 
lie in the same straight line as the thoracic sterna behind them, but aie 
directed upwards, so that a line drawn through them forms an angle of about 
140° with a line drawn through the hinder ends of the more posterior 
thoracic sterna. 
In the Eaninidse, as shown in fig. 17 for JS^otopus, a line drawn through 
the seventh, eighth, and ninth sterna forms an angle of 134° with a line 
drawn through the tenth and eleventh sterna. In Dromia the corresponding 
lines meet at an angle of 108°. This is another character in which the 
Dromiacea have departed further from the Astacuran type than the Raninidse, 
but, as regards the six post-oral segments, it would not be quite accurate to 
say that they have advanced further towards the cancroid type. There are 
certain points of resemblance between the Dromiacea and the Raninidse 
which may be taken as evidence of but slight modification from the common 
Astacuran stock from which both have descended : not, however, as evidence 
that one group has descended from the other. The evidence so far tendered 
shows that the Raninidaj in their nervous system and in the characters of the 
endophragmal skeleton are much more nearly akin to the Astacura than are 
Dromiacea ; they are, therefore, the more primitive group and cannot have 
descended from the less primitive. In both groups the departures from the 
Macruran type as exhibited in the first six post-oral segments are due to the 
acquisition of a broad sternum in the tenth segment and the upward cant of 
the sterna immediately in front of it, with which are associated the retreat 
of the sub-oesophageal ganglion mass into the thorax (see fig. 9 for Raniiia)^ 
the decrease in length of the carapace and the increase in width of its 
anterior margin. The mouth is also brought to a more anterior and less 
ventral position than in the Macrura. 
