BLUE1SH HYDJIUS. 561 



of the vent is not so strongly marked as in the 

 great Hydrus, to which this species appears allied 

 in some particulars. It is preserved in the Bri- 

 tish Museum. 



BLUEISH HYDRUS. 



Hydrus Caerulescens. H. c(trukscens,fasciis decurrentilits cceru- 



Ids, abdomine albo. 

 Blueish Hydrus, with dusky-blue decurrent bands, and white 



abdomen. 



LENGTH two feet : habit resembling that of the 

 Great Hydrus, but the abdomen has a single and 

 perfectly undivided row of hexagonal scales, from 

 the throat to the beginning of the tail, of about 

 the tenth of an inch in diameter, and forming a 

 flat carina on that part : back marked by a carina 

 also, but the scales not differing in shape from those 

 on the rest of the body, being hexagonal, with an 

 abrupt middle carina 1 head not broader than the 

 neck, and covered with large scales: tail two 

 inches and a quarter long, and of the usual form 

 in this genus, being moderately broad, but, as in 

 the preceding animal, not equalling the thickest 

 part of the body. Colour pale livid blue above, 

 and white beneath, but marked throughout its 

 whole length, as in the preceding species, by de- 

 current fasciae of a deeper blue, and which dip on 

 the white of the sides : they commence immedi- 

 ately from the head, the top of which is of a simi- 



