i86 Report S.A.A. Advancement of Science. 



all the way round, and is formed by the marginals being inclined 

 at an angle with the costals, emphasized by a certain nodulation of 

 the upper part of the marginals. In other sub-groups the angulation 

 is altogether wanting, the surface of the carapace being unin- 

 terruptedly smooth along the junction of the two longitudinal series of 

 plates. Between these two extremes all transitional variations are 

 to be found. 



The angulation seems to be most prominently and consistently 

 developed in trimeni, and the marginals here constitute a conspicuous 

 swollen border to the carapace. In tentoria the character is often as 

 well marked as in trimeni, but in some individuals it has nearly dis- 

 appeared. The verremtxii-<gxo\y^, as in so many other features, is 

 decidedly intermediate ; the angulation is rarely so marked as in 

 trivteni and tentoria, and is sometimes wholly wanting ; even one side 

 may differ considerably from the other. 



Geometrica and ocidifera are very pronounced types of the 

 smooth condition, the costal shields passing almost insensibly into 

 the marginals and continuing at the same inclination. Old and young 

 specimens, however, differ a little ; in the latter there is usually some 

 suggestion of angularity, which disappears with age. 



From the above brief mention of the condition of the margino- 

 costal angle in the various sub-groups, it is manifest that a complete 

 gradational series of variations exists between the two extreme types. 

 A certain degree of fixity in extreme angulation is presented by 

 trimeni and tentoria, verreauxii is remarkably fluctuating, and 

 ocidifera and geometrica are practically constant as regards their 

 smoothness. 



/. Nuchal Shield. 



The size and shape of the nuchal shield are characters which 

 have been found to be of diagnostic importance within the geometrica- 

 group, and justly so were the extreme variations alone to be taken 

 into consideration ; but transitional series are forthcoming here as in 

 the other characteristics, and indicate that though the structure may 

 be permanently fixed in certain types yet in no one sub-group has it 

 become sharply separated from others. 



The shield is least developed in tentoria and largest in ocidifera. 

 In the former species it may be so insignificant as to be invisible from 

 above, but a rudiment may be seen on the under surface ; in others 

 of the same species it is seen from above as a narrow border to the 

 first neurnl shield, and in a few it is longer than broad. In the 

 forms embraced under trimeni the nuchal shield is at about the same 

 stage of development as in tentoria. 



In all the members of the verreauxii sub-group the nuchal shield 

 is better developed than in most tentoria, though of similar form ; 

 as seen from above it appears as a nearly rectangular projection from 

 the front of the first neural, somewhat longer than broad, and usually 

 a little broader behind than in front. 



