54 F. G. Percival—Puncetation of Terebratulid Shells. 
were averaged. In order to keep approximately to the same area in 
each case a circle was drawn on each specimen. 
The range in each of the two species was very great. There is no 
reason to suppose that they are in any way abnormal. In 7. biplicata 
one individual had a density of only 39 per square millimetre, and 
one was as high as 129. 7’. punctata ranged from 66 to 240. Thus 
these two species alone cover a great part of the whole range of 
variation possible for the genus, and the feature must be of extremely 
little value for purposes of specific distinction. The following tables 
summarize the results. It was necessary to group the density 
numbers in tens for two reasons: first, the actual number of 
individuals was small compared with the great amount of variation, 
and secondly, certain numbers tended to predominate owing to the 
method of counting. In order to count accurately from 100 to 200 
tiny spots within a square of 14 millimetres one must, if 
possible, get them arranged in rows parallel with the sides of the 
square. As a result one often gets, say, eight rows of eight or 
nine puncte, and so on; i.e. numbers like 64 (8 X 8), 72 (8 xX 9), 
_ 81, 90, 100, 121, etc., will unduly predominate. So those individuals 
with a density of from 41 to 50 were grouped together, and so on. 
TABLE I.—T. pwnctata. (See Fig. 1.) 
Puncte per sq. mm. Nos. of individuals. 
61- 70 5 6 5 js . 5 ; ih 
71-— 80 0 a 4 : : ; : 3 
81- 90 A ° 6 : B : 0 9 
91-100 : 5 5 5 : : : 19 
101-110 i 3 : ‘ 5 : 6 32 
111-120 A 3 0 i 7 5 6 56 
121-130 6 fs 5 is - a , 56 
131-140 . A 2 ; 5 A c 46 
141-150 3 6 : , 5 * fe 44 
151-160 . . $ 3 : & é 41 
161-170 5 5 ¢ A 6 i 27 
171-180 ‘ s : é : a A 11 
181-190 : 5 - 4 - : 7 
191-200 A F 3 ‘ 4 
201-210 4 
211-220 : A é é ; : si 4 
221-230 5 3 : 6 , : 3 2 
231-240 ili 
Total . 5) BT 
TABLE II.—T. biplicata. (See Fig. 2.) 
Puncts per sq. mm. Nos. of individuals. 
31- 40 i iB 5 c $ A ; 1 
41- 50 é é 6 5 s 4 ; 13 
51- 60 4 5 5 3 5 4 36 
61-— 70 : 3 : : 3 : se 413} 
71- 80 b z i ; ‘ ; 41 
81-— 90 6 ‘ 3 é i : 5 14 
91-100 4 : A 5 3 é 10 
101-110 3 x A ‘ : s ‘ 6 
111-120 F A i 4 : A 1 
121-130 A . 5 5 , 4 4 ih 
Total ‘ = 6G 
