53 



clays belong to the orders Atremata and Neotremata, both of which Mostly of the 

 have representatives in the oldest Cambrian beds. In both, the mate and" 76 

 pedicle must have had much durability, as it succeeded in holding Neotremata. 

 the shell in position in a majority of cases until the latter was 

 weighted down with the accumulating sediment falling from the 

 turbid water. 



The pedicle is an organic part of the animal and is composed of Importance of 

 layers of chitinous and fleshy matter and is liable to decay on the relation'to 8 

 death of the animal. Orientation presupposes that the pedicle lasted orientation, 

 long enough to hold the shell in position until it was buried. The con- 

 ditions in the Etcheminiari beds show that the dorsal valve after the 

 death of the animal and the decay of the muscles and ligaments might 

 float off, and yet the ventral value would be held firmly in its place, 

 presumably with the aid of the pedicle. All brachiopods do not have 

 pedicles of equal strength and durability ; this is clear as regards the 

 Etcheminian forms, for some genera show greater susceptibility to 

 orientation than others, and it may be noted that it is the larger 

 species in which the orientation is more apt to be obscure. Outside 

 of the influence of the pedicle, form seems to have a good deal to do 

 with the attitude of the valves. This is manifest from the position 

 assumed by shells of the genera Obolus* and Leptobolusf in certain 

 layers of the Upper Etcheminian, for while only 33 p.c. of the valves 

 of the former are oriented, 81 p.c. of the latter have yielded to the 

 influences causing orientation. No genus shows more perfectly the Acrothyra 

 influence of form on orientation than AcrothyraJ entombed in the oriented, 

 same beds with the above genera ; in this genus 84 p.c. of the funnel- 

 shaped ventral valves are affected by orientation, while this phenome- 

 non can scarcely be traced in the attitude of the saucer shaped dorsal 

 valves. 



Leaving out of view the influence of the pedicle in holding the 

 valves in a certain position while being entombed, it is easy to see that 

 form has much to do with the phenomenon of orientation. We have 

 seen how diverse in shape were the two valves of Acrothyra, and how 

 differently the two valves acted in the process of burial, and that while 

 the ventrals exhibit a high percentage of oriented valves, the dorsals 

 show scarcely more than the natural quarter of their number so placed ; 

 we may on the contrary see how readily the valves of Leptobolus have 

 responded to the causes producing orientation. In this genus both 

 valves are oval and nearly alike, and this contour would^ favour the 

 placing of the valves lengthwise in the line of the current of water in 



* Obolus lens. f Leptobolus atavus. J Acrothyra proa via. 



