additional notes on the CAMBRIAN OF CAi>E tlRETON 
The series of Acrotreta run in size about as follows : 
Size and Form of the Ventral Valve in Species of Acrotreta 
OF THE Cambrian and Ordovician. 
Horizon 
OR 
Group. 
Coldbrook 
Group, 
Lower 
Etchminian, 
Upper 
? Etchminian, 
Probolenus 
Fauna, 
Lower 
Paradoxides, 
Upper 
Paradoxides, 
? Dolgelly, 
Uolgelly, 
Tremadoc, 
Tremadoc, 
Arenig, 
Llandello, 
do. 
Etage D, 
In Millimetres. 
0 
Namk and Reference. 
Length 
Width. 
Height. 
Proportion 
Wi<lth to 
Ileisht 
A. papillata — prima, PI, xv, Hgs. la to c 
2 
2.^ 
H I 
1 2. ‘ 
A. papillata, PI. xv, figs. 2a to f 
2^ 
8 
n \ 
2.4 
A. gemma, (Bill.) Walcott* 
8 
2 i 
1.0 
A. gemmula,! PI. xv, figs. 4a to d. .. . 
H 
H 
1 
1.5 
A. Baileyi:]: PI. xvi, figs, la to d 
:u 
4 
’ + 1 
3. - 
A. socialis, v. Seebach,§ 
8 
3 
2 ' 
1.5 
A. gemma, (Bill.), Walcott* * § , 
8 
8i 
01 
“'2 
1.4 
A. bisecta,li PI, xvi, 2a to g 
8 
31 
01 
"2 
1.8 
A. cf. socialis (Seeb), Brogger** * * §§ 
2 
u 
1.3 
A. sipo, n. sp. , Pi. xviii, figs. 1 and 2.. 
A. gemma, Billings, ! i 
If 
2 
2 i 
1. 
A. subconica, Kutorga,Jt 
4 
4 
^ 1 
.85 
A. Nicholsoni, Davidson, ^55 
4 
4 
H 
.94 
A. babel, Barrande, nil 
2 
2 ^ 
21 ! 
1.1 
It will be noticed that not only are the later species as a rule 
larger, but they are proportionately higher. Also we may observe 
that there were two lines along which there was a divergence in the 
relative height of the ventral valve, A. Baileyi had low umbones and 
approximated in form to Linnarssonia, a genus which, so far as has 
been observed, appeared in the Canadian Cambrian a little before it 
The other line of development culminated in the high umboned 
species of the Ordovician faunas. 
* Bull, U. S., Geol. Survey No. 30 p. 98. ■ 
t Trans. N. York, Acad. Sci.. No. XXVIII, p. Wi. 
t Trans. Roy, Soc , Can.. Vol. III., Sec. IV, p. 36. 
§ Brachiopoda Paradox. Beds, Sweden, Linrs. p. 16. 
t Nat. Hist, Soc., N. B., Bull, No. XIX, p. 275. , 
Die Silurisch. Eltagen 2und 3, p. 46. 
*t Paleozoic Fossils, Vol. I, pt. 1, p. 216. 
7+Monog. Br. Brachiopoda. Vol. 1, pi, IX. Fischer, Conchyliologie p. 1266. 
§§ Monog. Br. Brachiopoda, Vol I. pi. XVI, Vol. Ill, p. 338. 
IID Syst. Silur. Bohem, Vol. V, pi. 95. 
