[MATTHEW ] CAMBRIAN FAUNAS 99 
No. 6. DEVELOPMENT IN SIZE OF THE INARTICULATE BRACHIOPODS 
OF THE BASAL CAMBRIAN. 
In comparing the Brachiopods of the Coldbrook Terrane (Base 
of the Cambrian) with resembling forms of the next terrane above, 
the writer observed that in all the genera the resembling forms were 
larger. This had been found to hold in the genus Acrotreta, which 
in the range of time from the beginning of the Cambrian to the 
Lower Ordovician increased considerably in the height of the ventral 
valve, and also in the bulk of the valves generally. 
That this enlargement of the valves did not affect one genus 
only, but was notable in several, seems to indicate that there is a 
law of general application in the development of the Brachiopoda 
in this direction, such as is seen to have acted in the case of the 
Horse and of other Tertiary Mammalia. 
Owing to the smallness of these early Brachiopods this peculiar- 
ity in the forms of the successive faunas is easily overlooked, but 
in reality the change of bulk is quite noticeable, and in some cases 
is nearly as great as that observed in the species counted in the 
descent of the Horse from Hyracotherium to the modern Horse. 
The enlargement of the species of Acrotreta from the first that 
appeared in the base of the Cambrian to their culmination in Ordo- 
vician Time is described in some detail in a paper by the writer, con- 
tributed to the Bulletin of the Natural History Society of New Bruns- 
wick, last year. 
INCREASE IN SIZE OF THE VENTRAL VALVES OF ACROTRETA IN THE CAMBRIAN 
AND LOWER ORDOVICIAN. 







ra a x £3 
TERRANE OR ASSISE NAME ° a TS RE 
4 EF |ae4|) 4 
mm mm 
Coldbrook terrane........ A. papillata-prima...... 2 24 2° 5° 
Lower Etcheminian.......| A. papillata....... Léa 24 3 2°4 75 
Upper Paradoxides ..... A. socialis, v. Seeb...... 3 3 15 9° 
Dolgelly Group........... A DISECHAN ities ARR ee 3 34 1°3 9°75 
Llandeilo Group.......... A. subconica, Kutorga.. 4 4 ‘85 | 16: 
Increase of the latest be- 
VOU Me nie de to CA DCR DC cick: haste EN SAME RAR LAINE ART FER R ammere eye 3'2 

In this table, the third column shows the proportion of the 
width to the height of the valve, from which it will be seen that the 
ventral valve became proportionally higher as time went on. 
Sec. IV., 1902. 5. 
