97 



SlZE AND FORM OP THE VENTRAL VALVE IN SPECIES OP ACROTRETA 

 OP THE CAMBRIAN AND ORDOVICIAN 



It will be uoticed 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 and main line of develop- 

 ment culminated in the high uinboned species of the Ordovician faunas. 



For information regarding several European species of Acrotreta I am 

 indebted to Mr. Gilbert Van Ingen, of the School of Mines, Columbia 

 College, New York. 



* Bui. U. S., Geol. Survey No. 30, p. 98. 



t Trans. N. York, Acad, Sci., No. XXVIII, p. 126. 



J Trans. Roy, Soc., Can., Vol. Ill, Sec., IV, p. 36 



Brachiopoda Paradox. Beds, Sweden, Linrs. p. 16. 



IT Nat. Hist. Soc., N. B., Bull. No. XIX, p. 275. 



** Die Silurisch, Etagen 2und 3, p. 46. 



tt Paleozoic Fossils, Vol. I, pt. 1, p. 216. 



Monog. Br. Brachiopoda, Vol I, pi. IX. Fischer, Conchyliologie p. 1266. 



Monog. Br. Brachiopoda, Vol. I. pi. XVI, Vol III, p. 338. 



Illl Syst. Silur. Bohem, Vol. V, p]. 95. 



7 C. R. 



