A FOSSIL ISOPOD OF THE FRESHWATER GENUS PHREATOICUS. 369 



duced at the postero-distal angle. The 6th peraeopod is 

 slightly longer than the 5th, but shows quite the same 

 structure, though the more distal joints are rather obscure. 

 The 7th peraeopod is still less perfectly preserved, but is 

 evidently longer than the sixth, apparently reaching back- 

 wards as far as the posterior margin of the 4th segment of 

 the pleon; only the distal joints are represented; the merus 

 does not appear to be expanded. 



Of the pleon, the first four segments can be made out, 

 particularly the downward prolongations of the pleural 

 portions, which are very distinct. Of these, the first is the 

 shortest or narrowest, and is not produced so far downwards 

 as the second, which is considerably broader than the first 

 and has its anterior margin rounded, somewhat as in the first 

 segment. The third segment appears about as long as the 

 second, and reaches downwards to about the same level, 

 both the anterior and posterior angles being rounded. The 

 impression of the 4th segment is less distinct, but it appears 

 to be about as long as the third, and produced a little 

 further downwards ; its posterior margin shews a slight 

 concavity in the middle portion. The remaining segments 

 are absent. The upper portion of these pleon segments 

 cannot be made out with any certainty, but the distinctly 

 marked pleura, as described, are so characteristic of 

 Plireatoicus that they alone would be sufficient evidence 

 that the impression is actually that of a Plireatoicus, and 

 this conclusion is abundantly confirmed by the well pre- 

 served peraeopoda. The impressions of the segments of 

 the peraeon are by no means clear, but they appear to 

 represent the five posterior segments and to correspond to 

 the peraeopoda already described; the dorsal margin is 

 fairly distinct, so that the depth of the segments can be 

 measured with fair accuracy. The depth of the peraeon 

 appears to be rather greater in comparison with that of 



X- October 3. 1917. 



