Manchester Memoirs, Vol. Iv. (191 1), No. 15. 13 



Etheridge,* as its umbonal markings are strictly diagonal. 

 It seems to be merely a Cretaceous derivative of 

 T. clavellata. 



Other Types of Diagonal Ornament. 



The Scnphoidece [Lycett] are diagonally marked, but 

 differ from the ClavellatcB in not having the ornament 

 continued to where the flank bends round to face anteriorly 

 at least. Consequently, the diagonals form straight 

 lines {Fig. 6). The anteriorly-faced part of the flank is 

 usually marked by transverse bars. These may possibly 

 be due to the coalition of uplifted diagonals, as in the 

 anterior parts of the diagonals of T. clavellata, in which 

 case this group would be derived from the Clavellata 

 proper by loss of the anterior part of the flank ornament. 



In the Upper Lias of Robin Hood's Bay occurs 

 T. literata, which has been placed by Lycett in the Un- 

 dulates. This position is untenable, because its umbonal 

 markings are diagonal. 



Its flank markings, consist of well-marked diagonal 

 rows running to about halfway across the flank In 

 front of this are broken, more or less well-accentuated, 

 growth lines which make a seeming V in the central part 

 of the flank. On the anteriorly-faced part of the flank 

 these may become better marked. This type of ornament 

 seems to me to place it in the ScaphoidecB section of the 

 diagonally ornamented Trigonics. 



T. striata. This species is diagonally ornamented but 

 differs from the others of this type in having spinous 

 serrations instead of tubercles on the diagonal lines. A 

 similar variation is seen in many of the Scabrce. What 

 relationship, if any, there is between the two I do not 

 know, but I hope to do further work on this. 



* Etheridge, 1881. "On a new species of Trigonia from the Purbeck 

 Beds of the Vale of Wardour." Q.J. G. S., vol. 37, p. 246. 



