1 06 Chionaspis. 



which communicate with short, stout, cylindrical ducts. A few similar pores 

 open on definite marginal prominences, of which there are— in typical forms — 

 one on the second, and two on each of the third, fourth, and basal spaces. The 

 circumgenital glands, when present, are normally in five groups. In one 

 abnormal species {simplex) seven groups appear. 



It will here be convenient to correct an error that appears in Part I. (p. 31). 

 It is there stated— of the genus Chionaspis —'C!\2X the anal is usually slightly 

 above the genital aperture. More extended observation shows that their 

 relative positions vary. The genital orifice occupies an approximately central 

 position on the ventral surface of the pygidium, and the anal aperture may be 

 situate either slightly above or below it on the opposite surface. The same 

 correction must be made for the genus Diaspis. 



For purposes of comparative description the pygidium of typical Chionaspis 

 may be marked out into six difterent tracts (PI. XXXI. {fig. 4, A-F), which I 

 believe will be found to approximately correspond with the areas of suppressed 

 segments. There are no really definite lines of demarcation to be found on the 

 natural object ; but we are assisted in determining their position partly by a 

 few interrupted chitinous thickenings of the body wall, by the direction of the 

 superficial folds, and by the series of oval pores. Fig. 4 is a diagrammatic view 

 of the dorsal surface. The series of oval pores, judging from their position on 

 the abdominal segments, may be taken as indicating the posterior border of 

 each tract. The intersegmental line cuts the margin at the incision immediately 

 anterior to each squame or group of squames. Starting from the centre, the 

 mesal space (A) includes the anal aperture, and tapers downwards to a point 

 between the median lobes. In species where the median lobes are widely 

 separate there is sometimes a pair of minute squames or spines on the margin. 

 The margin of the first lateral space (B) carries one of the median lobes, a 

 spine, and one or more squames. The second lateral space (C) has a pore- 

 bearing prominence, the first lateral lobes, a spine, and one or more squames. 

 The third lateral space (D) has the same marginal character as the second, with 

 the addition of an oval pore at the base of the lobes — between the lobules. 

 The fourth lateral space (E) has two pore-bearing prominences, a third smaller 

 prominence, a spine, and one or more squames. The fifth lateral, or basal 

 space (F), is similar to the preceding, but broader at the margin, and bearing a 

 larger group of squames. The series of oval pores are broken into two groups 

 on each side, and are usually confined to the third, fourth, and fifth spaces, 

 though they occasionally occur on the others. 



The divisions of the ventral surface are not so readily made out. The 

 demarcation in the diagram (Plate XXXI., fig. 5) is more or less tentative. 

 The mesal space (A) is taken as consisting of a narrow tract, bounded above by 

 the genital orifice and impinging upon the margin between the median lobes. 

 The first lateral space (B) includes the median group of circumgenital glands. 

 The second lateral space (C) encloses the anterior lateral groups. The third 

 lateral space (D) carries the lower lateral groups. On the fourth and basal 

 spaces (E, F), and often on the second and third abdominal segments, are more 

 or less definite glandular scars, irregularly reticulate or lobulate. I believe these 

 latter organs to be homologous with the grouped circumgenital glands. In an 

 e.xample of Ch. hedyotis 1 have in one instance found the glandular scar on one 

 side of the fourth space replaced by a supernumerary group of normal glands. 



