52 F. G. Percwal—Punctation of Terebratulid Shells. 
Where the shell is smooth one may find either a square or a quin- 
cuncial pattern, but this regularity is broken by the subdivision of 
the rows, both transversely and radially. There is a tendency here 
and there for the lines to become very irregular, the radial rows 
‘streaming out’ into many branches. This occurs, Mr. J. W. Jackson 
informs me, particularly along the muscle impressions. In the 
gerontic stage the transverse lines are no longer in sweeping con- 
tinuous curves, but are broken up into many little wavelets, and it is 
very difficult to count them. 
With regard to the size of the puncte, one can distinguish roughly 
between coarsely and finely punctate shells, but nicer distinctions 
are unreliable, since the shapes of the individual puncte depend 
very largely on the amount of wear the shell has suffered; e.g. in 
T. punctata, Sow., an unworn shell may show long slit-like puncte, 
while a more worn test has rounded pores. Usually the puncte are 
hollows, but in specimens that have been naturally etched the 
material filling the puncte may project beyond the general surface 
level. Sometimes the ‘walls’ of each punctation project slightly, 
forming a small tube-like process. But all these differences may 
occur in one species, or even in different parts of one individual, and 
are therefore useless for our purpose. 
The only other convenient means of distinction by puncte is the 
variation in the number per square millimetre (or ‘density’). If 
the various species have sufficiently different densities this would be 
a valuable aid in specific determination. It is already so used to 
some extent." 
There is a gradual increase in the density from the umbo outwards 
to the anterior edge. The following readings are typical— 
T. biplicata (Brocchi) (a) 93, 111, 115, per sq. mm. 
(6) 72, 75, 86, 98, per sq. mm. 
(c) 68, 62, 81, 92, 112, per sq. mm. 
(d) 49, 54, 67, 90, per sq. mm. 
T. aff. crickleyensis, 8. Buckman, 224, 292, 344, per sq. mm. 
In each of the above five examples the first reading was at about 
1:5 cm. from the umbo, the other numbers being progressively 
farther out. Not all of the individuals show such big variations, and. 
this increase is not always regular. Occasionally there will be 
a decrease in: density over a zone a few millimetres wide (see 
example ¢ above). Again, when a strongly marked growth-line 
intervenes there is usually a sudden change of density. This is to 
be expected if there is a continuous variation from youth to age, 
a variation which is maintained even during the period of slow 
growth or cessation of growth represented by such a strongly marked 
growth-line. In 7. diplicata (Brocchi) the rate of increase is more 
rapid towards the sides than towards the anterior along the median 
line. This is obviously because the shell is longer than it is broad; 
hence growth takes place more rapidly at the anterior edge than at 
1 See e.g. F. Blochmann, Die Brachiopoden der Schwedischen Siidpolar- 
expedition, Stockholm, 1912, and J. W. Jackson, ‘‘ The Brachiopoda of the 
Scottish National Antarctic Expedition ’’; Trans. Roy. Soc. Hdin., vol. xlviii, 
pt. ii, No. 19. 
