REPORT ON THE ISOPODA. 43 



abdomen into a single plate which covers the gills ; the extreme length of the thoracic 

 appendages, which are modified to form clawed ambulatory limbs, as well as the gradual 

 decrease in size of the three posterior thoracic segments, are quite in harmony with this 

 identification, which appears to me to be the only one possible. 



The fact that in both cases the body had been broken off at the fourth segment of the 

 thorax, suggests that the anterior segments of the thorax were in all probability broader 

 and shorter than those # which follow; if this suggestion prove to be correct the species 

 only presents us with an exaggeration of the form of body characteristic of such species 

 as Ischnosoma spinosum. 



I do not wish to deny that both Ischnosoma bacillus and Ischnosoma bacilloides 

 may ultimately be shown to be in reality identical ; they however present sufficient 

 differences to warrant their distinction as two species, supposing of course that these 

 differences do not prove to be sexual. I am not aware of any other allied form in which 

 such secondary sexual characters are developed. 



The single specimen of Ischnosoma bacillus has been mounted in Canada balsam on 

 a slide ; it measures 10 mm. in length by 1 to 2 mm. in breadth. The length of the 

 several segments which compose the fragment is as follows : — Fourth thoracic 2 - 5 mm., 

 fifth thoracic 4 mm., sixth and seventh together l - 25 mm., caudal shield 1"25 mm., 

 the above measurements give, at any rate, the right proportion between the several 

 segments. 



The fourth segment of the thorax (which unfortunately is the anterior end of the 

 specimen) is like the rest cylindrical in form, widening out somewhat both at its 

 anterior and posterior extremities ; more particularly is this the case with the anterior 

 region of the segment which, as shown in the figure (PL VI. fig. 6) is of considerable 

 breadth, nearly as broad in fact as the segment is long ; the whole segment has, there- 

 fore, the shape of a T ; the two " arms " of the T are broad at their bases, where the 

 limbs are articulated, beyond the articulation of the limbs they suddenly narrow into a 

 long spine slightly curved forwards ; these no doubt represent the epimera. 



The fifth segment of the- thorax is, as the above measurements imply, considerably 

 the longest ; the comparative elongation of this segment is not remarkable, because in 

 the other species {e.g., Ischnosoma spinosum, PL VI. fig. 1) the fifth segment of the 

 thorax is also the longest. Its form resembles that of the preceding segment, only 

 reversed ; the " arms " of the T are at the posterior instead of the anterior extremity of 

 the segment, they are equally long with those of the fourth segment, but the epimeral 

 spine appeared to me to be straighter and not so bent as in that segment ; at the point 

 where the fourth and fifth segments articulate with each other the diameter of each is 

 slightly wider than in the middle of the segment. 



The sixth and seventh segments of the thorax are very short indeed, and together 

 about equal the caudal shield in length. The sixth segment is the longer of the two ; 



