4 March, Moi-phogenesis of certain Pdccypoda. 



the fact that anterior growth is slower than posterior, as 

 is natural in an animal which has to plough its way through 

 the mud. An increase in the rate of the current in which 

 they lived would produce a decrease in pre-umbonal 

 development, and so tend to throw the umbo forward in 

 forms living in strong currents. 



Usually rivers are far swifter than canals. But the 

 canals in which these elongated forms occur have steep 

 gradients, and therefore many locks. Strong lock currents 

 are felt at any rate near the locks. The fact that these 

 forms occur in canals with strong gradients, the more 

 modified ones in the steeper and more numerously locked 

 canals, seems to indicate a correlation between the two 

 facts, although I cannot ascertain the exact locality,* with 

 regard to locks, in which the specimens occur, nor the 

 distance to which the lock currents are felt. Current 

 action has been shown to be responsible for the curious 

 " Platiform " varieties from the river Foss and Lake 

 Rudyard. {Figs. D & E of Plate>, 



Elongation is also produced by the quality of the mud 

 in which they live, thick mud inducing elongation. But 

 forms on a similar mud to the Burnley and Marple ones 

 are not elongated as they are. 



3. The relation between current and form accounts in 

 part for the Keuper forms. The rivers of these districts 

 are slow and so lead to the development of forms with a 

 long dorso-ventral axis. It does not, however, account 

 for the thickness of the shell, or the greater lateral axis. 

 These must be due to the composition of the mud in which 

 they are found. 



It is a remarkable fact that the shells examined from 

 districts highly charged with Ca CO3 have thin shells and 



* The majority of the Marple specimens were obtained from the lock 

 basins themselves or close to the locks. 



