54 



which they lived, as being the position of least resistance. So it 

 happens that both dorsal and ventral valves can be included in the 

 record of orientation, without materially affecting the percentage ; 

 while to follow the same plan with Acrothyra would reduce the per- 

 Leptobolus centage of oriented valves nearly one-half. Yet we notice that in 

 oriented. Leptobolus there is a much larger proportion of reversed valves than 

 in Acrothyra ; that is of valves having the longer axis parallel to the 

 direction of the flow of the orienting current, but with umbo, in 

 place of the front of the valve, pointing in that direction. These 

 reversed valves we regard as affected by orientation, but not oriented. 



Their abundance in Leptobolus as compared with Acrothyra may be 

 dae to one of two causes. In Acrothyra the umbo was strong and 

 heavy and in cases where the pedicle had perished, and the ventral 

 valve sank to the bottom the point of the shell would touch first and 

 the shell would swing on this as on a pivot, presenting its smallest and 

 heaviest end to the force of the current. Another cause which might 

 have helped to cause the diversity in the attitude of the valves of 

 the two genera is that the umbonal region of the ventral valve in 

 Leptobolus was thin and the pedicle correspondingly attenuated ; if 

 it perished readily the valve would be sooner at the mercy of the 

 current, and so would orient with either end presented to the current, 

 indifferently. 



Obolus scftrce- ^ n Obolus we note a genus which resembles Leptobolus in the incon- 

 ly affected by 8 pi CUO usness of the umbo of the ventral valves, but differs in having 

 flatter valves which are circular ; and although the valves are thicker 

 and heavier, the weight is somewhat evenly distributed. These nearly 

 round saucer-shaped valves have left but a very imperfect record of 

 orientation, for in the lot examined one-third were oriented and 28 

 per cent reversed, thus 39 per cent show entire indifference to the 

 course of the orienting current. 



There are a few valves of Acrothele, but they are insufficient to be 

 of value in this question of orientation. But, having in view the two 

 genera first discussed, I think these clearly show a current passing in 

 a north-east direction through Indian Brook valley when the Etche- 

 minian terrane was being formed. At the same time there seems to 

 be abundant proof of shore lines near at hand from which felsitic stones 

 and sand were derived, and from which the felspathic sand and mud 

 were swept that rapidly entombed these shells. 



