Manchester Memo i is, Vol. Iv. (191 1), No. 15. 7 



them with the Trigonioe, the range of variation in the 

 latter must be far smaller in Juyassic forms at least, This, 

 however, does not entirely remove these factors from 

 affecting the Trigonia;, though it lessens their apparent 

 effect. One factor is probably removed, and that is current 

 action. 



Recent Trigonue are found from about 10 to 20 

 fathoms, a depth at which currents would not be felt. 

 There is no reason to suppose that Jurassic TrigonicB 

 lived in shallower water, so that their elongation cannot 

 be due to current action. It may be due to the consistency 

 of the mud in which they lived. This supposition is 

 supported by the fact that the elongated forms occur in 

 the Oxfordian and Kimmeridgian clays. These were 

 formed under similar conditions from heavy muds, which 

 would tend to retard anterior growth, and accelerate 

 posterior growth, of any animal crawling through them. 

 It is true that T.juddiana is found in the same formation. 

 It occurs, however, locally, and may be due to local con- 

 ditions, as are short forms among otherwise elongated 

 individuals in the UnionidcB. It might, perhaps, even be 

 due to local conditions of food supply, which, in doing 

 away with the necessity for much movement, would 

 reduce the effect of the environment on the shell. 



A tendency towards the development of an equilateral 

 form would be due to the assumption of some habit, 

 which, more or less, equalized the effect of the environ- 

 ment. 



Such a form in the modern TrigonicB and in Pecten is 

 associated with the fact that they do not crawl but leap. 

 Some such an equalizing habit must have been adopted 

 by T. dcedalea and its allies. 



This variation in form and consequent variation in 

 ornament may account for many of the so-called specific 



