THE GIRDLE SCALES OF ISCHNOCH1TON. 
The writer examining the type found that there had been 
some inaccuracy in the description of the girdle scales. They 
are very small, the length and breadth being about 36 /t *, with 
extremes varying from 30 to 40 /l They are rounded or 
slightly elongate, a little uneven in size, and moderately closely 
set. They are not smooth, but are striated, the striations being 
fine, even, numerous (20 to 30), and beautifully engraved 
(Fig. 1). An occasional pointed form is met. Two such occur 
in that portion of the girdle from which the figure was drawn. 
Although they imbricate, they do not show that close regular 
pattern which is found in most members of genus Ischnochiton. 
The peculiar shape and delicate striation, both so unlike 
any other Ischnochiton, offer some justification for the subgenus 
Chartoplax proposed by Iredale and Hull. 
Although it is possible to raise technical objection to the 
separation of genus Haploplax from genus Ischnochiton, there 
can be no question that members of the former genus constitute 
a well defined sectional group. Six Australian species have 
been described, H. smaragdina Angas, H. resplendens Bednall 
and Matthews, H. lentiginosa Sowerby, H. adelaidensis Reeve, 
H. thomasi Bednall, and H. arbutum Reeve. All may be recog- 
nized “at sight’’ as belonging to Haploplax. They are all 
broadly elliptic in shape, being proportionately wider than 
Ischnochiton generally. They are all of medium size, rarely 
exceeding 25 mm. in length, and being, as a rule, much smaller. 
The sculpture is weak, the surface often glossy, and the coloura- 
tion usually striking and characteristic of the species. The 
girdle scales are proportionately large and glisten like polished 
gems. Moreover all the members have a preference for a fairly 
high station in the littoral zone. 
Ischnochiton ( Chartoplax ) pur a has nothing in common with 
any of these characteristics except lack of sculpture. For the 
purpose of comparison, specimens of Ischnochiton crispus Reeve 
and Haploplax smaragdina Angas equal in length to the specimen 
of I. (C.) pura were examined and portions of the girdle were 
figured on the same scale as that of I. ( C .) pura (Figs. 1, 2, 3). 
It was found that scales of I. (C.) pura measured about 36 x 36,*, 
those of I. crispus about 160 x 60/», and those of H. smaragdina 
about 218 x 218 /l The scales of I. (C.) pura were thus less 
than a quarter the size of I. crispus and about one-sixth of 
H. smaragdina. 
Its station indicates that I. ( C .) pura is a deep water Ischno- 
chiton. It may be remarked that the insertion plates and 
* /t =l/1000 mm. 
[155] 
