4 March, ISIorphogcuesis of certain PcJccvpoda. 



Ccrnbrash is T. detrita which is a later variety of T. v-costata 

 and T. nndulata again, which also appears in the Corallian 

 with T. gecgrapJiica, a closely allied form to T. costntitla. 



So far as I know no Trigonine of this type occur 

 either in the Oxfordian or Kimmeridgian clays. But, as 

 these deposits were laid down under very similar physical 

 conditions, and as these forms occur before, between and 

 after them, it appears quite legitimate to assume that the 

 conditions under which these deposits were formed were 

 unsuited to these particular Trigoniae. 



In the Portlandian times these forms with concen- 

 trically marked umbones re-appear and show a rapid 

 development of ornament. 



The simplest type is T. Damoniana. Here it is neces- 

 sary to point out that two very distinct types are described 

 under the name " damoniana " in Lycett's monograph from 

 types both in Jermyn Street and in South Kensington. 

 In both the area is concentrically marked at the umbo. 

 These forms tend to lose the ornament on their areas 

 in ephebic stages. The flank ornament passes through 

 the curvirostris and costatula types during nepionic 

 stages. When adult the markings consist of concentric 

 costations which reach about half way across the valve, 

 and then break up into the usual alternate pustulations, 

 with, at least in one place, one pair of anterior costse to 

 one posterior row of tubercles. This marks the former 

 existence of the undulation, though it is no longer visible. 

 Its loss may, I think, be attributed to two causes. 



{a) The greater isolation of the tubercles, which thus 

 tend to lose their diagonal connection. 



ib) The loss of ornament near the marginal carina, 

 which leaves a clear space where the undula- 

 tion should be. 



