2 March, MorpJwgenesis of certain Pelecypoda. 



quincuncially arranged tubercles. Indeed this arrange- 

 ment alone gives a semblance of diagonal ornament 

 without any fusion of tubercles, or development of inter- 



Fig. Ml. J-'ig- "i-l'- 



Apparent development of diagonal Sliows the apparent development 

 rows owing to (juincuncial of straight lines of ornament 



arrangement of spots. from the ordinarily arranged 



tubercles. 



Fig. 2. 

 Showing the quincuncial airangement of the tubercles of T. claveUata. 



mediate ridges, as is seen in Fig. \a, where the lines 

 appear to run diagonally, not straight as they do in \b. 



