56 F. G. Percival—Punctation of Terebratulid Shells. 
This may imply the oncoming of a variation, but possibly it may mean 
that the mode of the specimens from this locality was a little lower 
than the mode for the species as a whole. If, for example, the mode 
for the whole species were 150, and the mode for this particular 
locality between 110 and 130, one would expect the numbers of 
individuals with a density of 130 to 150 to be greater than those 
with density 110 to 90, i.e. the curve would slope down more gently 
to the right of the mode than to the left. 
In conclusion three points may be emphasized: (1) The shapes of 
the individual puncte depend to a great extent on the state of 
preservation of the test, since the shape of the cross-section of a punc- 
tation often varies as it passes outwards, from circular to oval or 
slit-like. (2) There is in general a progressive increase in density 
from the umbo outwards, which is approximately the same in each 
valve. This increase is not always regular; occasionally the density 
will decrease for a while, and at the larger growth-lines there is 
usually a sudden increase. (3) The amount of variation in a species 
is so great as to make the density almost valueless as a specific 
character. 
I wish to offer my grateful thanks to Dr. A. Morley Davies, 
Dr. G. Hickling, and Professor H. H.Swinnerton for the advice and 
help they have given at various times during the rather tedious work 
on which this paper is based. 
EXPLANATION OF PLATE III. 
Fig. 1. Dyscolia crossei, Day. Recent: Japan. Middle of ventral valve 
photographed from outside, showing branching punctations. The 
forks point to the umbo. (This is the case in all Terebratuline 
I have examined, though Fischer & (hlert, in describing this 
species, state that the canals are directed obliquely ‘‘ d’arriére en 
ayant .)) x 485 
», 2. Microthyris sublagenalis, Day. Bathian: Marquise, Boulonnais. 
Dorsal valve, less than 1 em. from umbo, showing a sudden increase 
of density from 168 to 272 per sq. mm. x 20. 
», 3. LT. punctata, Sow. Middle Lias: Stathern, Leicestershire. Portion 
‘ of test showing ‘‘radial canals’’ at outer aperture of each 
punctation. x 40. 
» 4. TL. punctata, Sow. Flake of shell by transmitted light, showing puncte 
with circular cross-section, changing to slit-shape at the outer 
surface. x 40. 
» oO. TL. punctata, Sow. Test by reflected light. A thin outer flake is 
removed to show the round section of each punctation towards the 
inner surface of the test. x 14. 
REFERENCES. 
1, CARPENTER (Dr. W.). ‘‘On the Microscopic SUN: of Shells’’: Rep. 
Brit. Assoc., 1844, pp. 16-18. 
2. SHARPE (Daniel). ‘‘On Trematis’’: Q.J.G.S., Fol: iv, p. 67, 1848. 
8. SoLuas (Professor W. J.). ‘‘ Notes on the Cxcal Processes of the Shells 
of Brachiopoda interpreted as sense-organs’’: Sci. Proc. Roy. Dub. 
Soc., N.S., vol. v, 1886-7. 
4, Morse (E.S.). ‘‘On the Embryology of Terebratulina’’: Mem. Boston 
Nat. Hist. Soc., vol. ii, 1871-8. 
5. KiNG (Professor W.). ‘‘ On the Histology of the Test of the Class Pallio- 
branchia ’’: Trans. Roy. Irish Acad., vol. xxiv, p. 439, 1867. 
