BRACHIO POD A—THOMSON. 
0 
different species, Blochmann (1908), who has called attention to this character, figures 
the middle part of the ventral valve in front of the muscular impressions. It seems 
probable from the differences displayed by his figures that a study of the mosaic may 
prove of considerable value in distinguishing species which are similar in external form. 
Unfortunately this character can seldom be observed in fossils. The pores or tubules 
which penetrate the shells of many species also show considerable differences in certain 
cases. Blochmann makes measurements of the outer and inner diameter of the pores, 
and of the number per square millimetre, and recommends that the same district of the 
shell as mentioned above, viz., the middle of the ventral valve in front of the muscular 
impressions, be selected for comparison, and also that all figures be reproduced with a 
magnification of 100 diameters. For portraying the shell mosaic he has unfortunately 
chosen a magnification of 175, and in this has been followed by Eichler (1911) and 
Jackson (1912). As a figure of the mosaic of a punctate shell serves also as a figure of 
the pores, it is here recommended that a magnification of 100 be adopted for the figures 
of both structures. 
Blochmann was able to show from a study of the pores that the specimens from 
Kerguelen Islands formerly identified as Terebratella dorsata belonged to a distinct 
species. In other cases, such as the genus Liothyrina, the differences between the 
species are so slight that a study of the pores does not assist in identification. 
Percival (1916) has made a study of the punctation of certain English fossils, 
and especially the “ density ” or number of the pores per square millimetre, and finds 
that the range in the two species selected for close study is so great that this feature 
can have little value for specific distinction. He has certainly proved his case for the 
species studied, Terebratula biplicata and T. 'punctata, but it does not follow that species 
of other genera will show a similar behaviour. In the present collection the large and 
distinctive pores of Magellania joubini enable the young of this species to be easily 
distinguished from the young of other species of Magellania and Macandrevia, the 
latter having always relatively small pores, and it appears probable that a fuller use of 
these characters may prove of considerable value in other cases. Meanwhile it is 
desirable to have figures of both pores and mosaic of as many species as possible. 
For counting the number of pores, Blochmann uses a netz-mikrometer. Percival 
adopts a camera giving a magnification of fourteen times, and isolates an area of 14 
sq. nun. on the screen by cutting a hole of this size in a sheet of paper. In the absence of 
an ocular micrometer I have found the following a convenient procedure. A Dick-Swift 
petrological microscope is used, the tube and the eye-piece of which are slotted 
to receive certain accessory plates. In this slot is inserted a piece of Bristol board 
with a square cut out of it of such a size that it gives an area of 1 sq. mm. on the object 
with a given objective (1 inch). The size of the square is determined experimentally, 
using an ordinary stage micrometer for calibration. The data obtained by this method 
are lower in the same species than those obtained by Blochmann, but I have checked its 
accuracy by the camera method of Percival. 
*20218—B Vol. IV, Paet 3. 
