JManchestcr Memoirs, Vol. Iv. (191 1), No. 11. 9 



the radial ornament, running up to the umbo. The length 

 of these concentric lines is gradually reduced by the in- 

 troduction of radials. 



Fig. I. 



Diagram illustrating the umbonal and flank ornament of Recent and 

 Miocene Trigonias. GG = a growth line. 



This, then, gives the history of costate ornament from 

 early Triassic or pre-Triassic times to recent ; and shows 

 the close connection between it and the Undulate types. 



General Considerations on Ornament. 



Beecher has shown that ornament is due to special 

 body activity. As it is the mantle edge which secretes 

 the shell in the Pelecypoda, the ornament must be due to 

 extra activity of the mantle edge, as a whole, at intervals 

 of time. Possibly these time intervals coincide with the 

 variation of the seasons, the ornament being developed 

 in summer when the body activity as a whole is aug- 

 mented. The simplest kind of ornament will then be the 

 purely concentric. {Fig. 3 A.) 



The next stage to be developed will be that due to a 

 mantle edge which is especially active at definite points. 

 That is to say, the ornamentation will be still concentric, 

 but will also be tubercular. 



As shown by Beecher tubercles or spines are due to 

 the intersection of two lines of ornament, and these 



