igiô. No. lO. THE LOWER CAMBRIAN HOLMIA FAUNA. 87 



Callavia euc/iaris in particular appears to point back to a XcvcKÙ'a-form as 

 its probable ancestor. 



Finall}', we must oppose Walcott's hypothesis that the Middle Cam- 

 brian family Paradoxidae developed from the above forms. This is undoubt- 

 edly a theor}- that has been applauded by most writers and which appears 

 very natural and obvious. The Paradoxidae undoubtedl}' by their form 

 and structure strongly remind us of several Lower Cambrian Mesonacidac. 

 and succeed them in point of time. 



The solution of a problem, that at first appears most natural is not 

 always the true one. In my opinion this is the case here. A careful 

 consideration oi this problem has convinced me that Walcott can scarcely 

 be right. 



In his frequently quoted work. Walcott says with respect to this 

 matter : — 



The question of the transition from the Lower Cambrian fauna to the 

 Middle Cambrian fauna is one that has not been fully worked out. That 

 all of the genera of the Mcsojiacidae should disappear before the undoubted 

 appearance of Paradoxides is a very significant fact, and to me indicates 

 that there was a transition fauna in the Atlantic Province, anc| that in most 

 instances owing to shifting shore lines and irregular deposition of sediments 

 the record is incomplete. Both in England and New Brunswick the Proto- 

 Icniis fauna has been found beneath the horizon of the Paradoxides fauna 

 and above the horizon of the Lower Cambrian fauna. The Protolenus fauna 

 has a comingling of generic types common to both the Lower and the 

 Middle Cambrian fauna, but as yet nothing has been found that could be 

 construed to be a connecting link between the Mesonacidae and Paradoxi- 

 dae. In the western Pacific Province fauna of China, India and Australia, 

 the genus Redlichia appears to be a form that combines characteristics of 

 both families, and it may be that Albertella may be found to have retained 

 some of the characters of the Mesonacidae, also Zacanthoides. The genus 

 'Albertella occours in the passage beds at the top of the Lower Cambrian 

 above Olcnellus canadensis ^. 



In another place he says : — The family Mesonacidae is distinguished 

 from the Paradoxidae mainl}- b}- the presence in the latter of tree cheeks 

 separable on the line of the facial suture from the cranidium. In the Mesona- 

 cidae the facial sutures are in a state of symphysis and the free cheeks and 

 cranidium are frequently not to be distinguished-. 



^ Walcott. Olenellus and the other Genera of Mesonacidae, page 250. 

 - Ibidem, page 253 



