12 March, MorpJiogenesis of certain Pelecypoda. 



This, I believe, to be formed from the tubercles of the 

 upper ends of lower diagonals. A possible explanation 

 of the development of a second line ma)' be seen on 

 Fig. 8c-. 



Here the radials (Ri, Ra, &c.) cut the concentrics 

 (Ci, C2, &c.) at an angle of 45'. The tubercles seen on 

 the figure are quincuncially arranged, as will be seen 

 from the following chart : 



+ represents a tubercle at the intersection of the concentric and radial. 



O represents the absence of such a tubercle. 



A blank indicates that the intersection is not visible on the diagram. 



On the figure it will be seen that the diagonal distances 

 D, even at so low an angle as 45", are over twice as large 

 as the distances d between the tubercles of succeeding 

 diagonal rows. The result is that on the diagram the 

 apparent lines of ornament run at right angles to the 

 concentric. On the umbo of T. dccdalea the crowding of 

 the growth lines, and the high angle at which the radials 

 and concentrics meet, accentuate this peculiarity and 

 result in the obliteration of the true diagonal lines, and 

 the seeming introduction of a second set of ornament. 



On this working T. dcedalea has the usual t}-pe of 

 clavellate marking but it is highly modified by the peculiar 

 form of the animal. There seems no need, therefore, to 

 put it into a separate section, as has been done by Lycett. 

 It certainly has no place in the Undiilatce^z.'s, suggested by 



