Alancliestcr Memoirs, Vol. Iv. (191 1), No. 15- 



XV. Studies in the Morphogenesis of certain Pelecy- 

 poda. (3) The Ornament of Trigonia claveilata and 

 some of its Derivatives. 



By Margaret Collev March, B.Sc. 



Read Marclt 21st, igir. Kercived for publication April 4th. igri. 



The original title of this paper was " The systematic 

 position of Trigo?iia irregularis, Trigonia scarburgensiSy 

 Trigonia dcedaka and Trigonia literata!' It was abandoned 

 for two reasons, yfrj-//;/ because it was too cumbersome, 

 secondly because that part of the paper appeared only 

 at the very end, and in a seemingly incidental manner. 

 I should like to point out that the curious ornament of 

 these forms and their relationships led to the working out 

 of that part of the paper which, in writing, had to come 

 first. 



The Ornament of T. claveilata. 



The valves of this species have been described by 

 Lycett* as having "rows of tuberculated costa;, at first 

 oblique, but the later-formed few become more horizontal, 

 or more nearly in accord with the directions of the lines of 

 growth, so that the greater number of rows reach the 

 anterior border in the form of attenuated or sub-tubercu- 

 lated varices ; the posteal extremities of the rows approach 

 the carina at an angle which is greater than a right angle. . .'' 



The rows of ornament are diagonal, since they cut 

 both radial lines and growth lines. They are formed of 



*Lycett, 1875. "Trigonia^," Pal. Soc, p. 19. 

 May jotli, igii. 



