176 Shimer — Lower-Middle Cambrian Transition Fauna. 



Art. XVIII. — A Lower-Middle Cambrian Transition Fauna 

 from Braintree, Mass. • by H. W. Shimer. 



Some time ago, while having a driveway excavated at his 

 home on Quincy avenue, in East Braintree, Mass., Mr. Thomas 

 A. Watson found a rather angular slate bowlder, about two feet 

 in diameter. He kindly turned it over to the Massachusetts 

 Institute of Technology. 



The slate is quite similar in appearance to that of the cele- 

 brated Paradoxides quarry on Hayward Creek ; it is similarly 

 metamorphosed but is lighter gray in color and lacks the pecu- 

 liar purplish tinge of the Hayward Creek slate. 



In it the following fauna was found : 



Name Abundance Previous occurrence Age 



Acrothele gamagei (Hobbs) r Hayward Creek Middle Cambrian 



Hyolithes shaleri Walcott r Hayward Creek Middle " 



Paradoxides harlani Green C Hayward Creek Middle " 



Strenuella strenua (Billings) R North Attleboro, Lower (i 



North Weymouth 



and Nahant 



? Strenuella strenua (Billings) R 



Olenellus (Holmia) broggeri Walcott R North Weymouth Lower Cambrian 



Ptychoparia rogersi Walcott r Hayward Creek Middle " 



Agraulos quadrangnlaris (Whitfield) C Hayward Creek Middle " 



C = very common ; c = common ; r = rare ; R = very rare. 



This fauna includes five species of the Middle Cam- 

 brian, two of which are very abundant, and two of the lower 

 Cambrian. There is thus a great predominance of the Middle 

 Cambrian element, though it indicates a persistence of the Lower 

 Cambrian element into Middle Cambrian times. So while we 

 have here a transition fauna, the rock must be assigned to the 

 Middle Cambrian period. This very interesting transition 

 fauna is the first recorded from this region to show in any way 

 a passage from the Lower to the Middle Cambrian. 



The size and angular character of the bowlder would indi- 

 cate that it had very probably not traveled far from the parent 

 ledge. Since the glaciers which transported it came from the 

 northwest, we examined the drift in that direction. Under 

 the leadership of Mr. Watson we found on the granite hill 

 south of Quincy Adams very many slate bowlders, some similar 

 in appearance to the one containing the transition fauna, though 

 no fossils were found in them. These decreased in number 

 southeastward ; on the granite hill they composed most of the 

 drift. Their great abundance here would indicate their prob- 

 able derivation from the drift-covered valley of Quincy Adams, 

 a suburb of Quincy. Some support likewise is given to this 



