



14 



2 I 



'9 



6 



2 I 



-3 



3—4 



2 I 



20 



5 



20 



2 5 



9 



19 



26 



8-9 



18 



19 



6 



17 



25 



6 



16 



2 1 



6-7 



i5 



23 



7 



15 



20 



8 



15 



20 



8—9 



1 — : 

 1—-. 



In the foregoing table the formulae for forty-two specimens have been printed separately, 

 so that future students may know what range to expect. The errors of observation are greater 

 in a species with few teeth and large opaque heads, than in the case of forms with numerous 

 teeth and small transparent heads; but the errors are, of course, not large enough to account 

 for the apparently wide range of variability. 



This extent of variation is however more apparent than real, for, by concentrating the 

 observations into groups differing by 5 mm. of total length, it is found that the range of variation 

 in the teeth is not much greater than in many other species ; in the jaws it is slightly greater. 



40 and 



more 



18—21 



4—7 



2—4 



2—5 



35 — 39 





20—23 



4-6 



2—4 



2 — 6 



30—34 





i7— = 3 



5-8 



1—4 



1—6 



25 — 29 





18—24 



5-3 



2—4 



2—5 



20 — 24 





18—25 



3—9 



2—5 



1—5 



15 — 20 





19 — 26 



6-9 



2—5 



2—7 



The point, in which ]iexaptcra really differs from other forms tabled in this report, lies 

 in the permutations in which the three varying parts of the armature are combined in one 

 individual, — more or fewer jaws being combined with more or fewer anterior teeth as compared 

 with fewer or more posterior teeth. I believe the explanation to lie mainly (perhaps not entirely) 

 in the length and slenderness of the teeth; many of them are probably torn out by the roots; 

 certainly many are broken off short, for their bases may be seen still in place. As a source 

 of additional weakness, the posterior teeth in older specimens often appear not to be attached 

 to the bony bar with which they are united in other species, but to lie at some distance from 

 it in a superficial plate of chitinous material. In the case of the jaws, again, the empty sockets 

 from which they have been torn are often recognisable. In addition to the above, it is possible 

 that, as Doncaster suggests, teeth may be shed with age. 



Characters. Head small when contracted, of medium size when expanded, in comparison 

 with the body; it is not marked off from the body by a neck when contracted. No collarette 

 present. Body flaccid, transparent in formalin; longitudinal muscles broad, but thinly developed; 

 lateral fields large. Trunk generally widest at about the middle of its length. Tail segment 1 7 

 to 26 per cent, of the total length. Ganglion small and rather far back. 



