40 BRITISH FOSSILS. 



Description. The general form of H. bilobus is elongate-oval in 

 front and attenuated behind (resembling a good deal the outline of 

 a Palcconiscus or other fish). The thorax is not easily distinguished 

 from the abdomen, into which it is attenuated, the greatest width 

 being about the third or fourth segment ; its segments are widely 

 transverse, those of the abdomen become less and less so, till the 

 last but one is nearly square ; the tail joint is oblong and emargi- 

 nate and narrowest of all ; the antennae are slender and long, the 

 swimming feet narrow, the palpi filiform ; and these general cha- 

 racters, taken together with the small size (seldom nine inches, 

 long), will easily distinguish the present species from the following. 

 The different portions may now be more minutely noticed. 



The carapace, one inch and a quarter in length, is half oval, only 

 one-fifth wider than long. The position of the long eyes is very 

 forward on the sides, and they somewhat interrupt the general oval 

 contour. They are broadly crescentic and convex, placed half 

 below (fig. 4 c) and half above the margin of the head, and their 

 extreme length is rather more than half that of the head. No 

 lenses can be seen with the naked eye, but when magnified the 

 appearance is that given in fig. 4 d. 



Body Segments. The body in well-preserved individuals is 

 barrel-shaped in front, the anterior or thoracic segments, taken 

 together, measuring about one inch and three quarters in length 

 by one inch and a quarter broad. They are widely transverse, the 

 anterior one being fully four times as broad as long, or even more. 



The second ring, as seen in a specimen not here figured, is five 

 times as wide as it is long, the central part gently arched forwards 

 in the middle, and at the sides the upper angles are produced for- 

 ward into blunt lobes,* the outer margins being oblique, and their 

 posterior angles are a little produced backward, particularly in the 

 sixth or hindermost thoracic ring. 



The remainder, which are here reckoned as abdominal, are gra- 

 dually narrower, the seventh being only three times as wide as long, 

 the tenth only once and a half, and somewhat narrower at the origin 

 than the posterior edge ; the penultimate^ (eleventh) is squareish, 

 only a little wider than long, also contracted at the origin and with 

 the outer angles produced to lap over the rounded anterior edges of 

 the caudal joint. A strong carina runs down this segment upon 

 the upper side, but for the lower half only. 



* This form of the second segment is common to all the species, see fig. 13 of this 

 Plate, and Plates IV., X., XIII., & c . 



