14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



in measuring them do not vary significantly from one point to an- 

 other, they are comparable at any given height with those of L. 

 hellaplicata (either subspecies) regardless of whether or not they 

 represent an equivalent stage of ephebic development. 



The absolute height of the smooth stage increases in progressively 

 younger species of the L. lugubris group (fig. 8), and its evolution is 

 characterized by transgression of its ventral margin from the neanic 

 to the ephebic part of the shell. The smooth stage terminates in the 

 early to middle neanic stage of L. hellaplicata novamexicana, and in 

 the middle to late ephebic stage of L. lugubris. Measurements of the 

 height of the smooth stage used in plotting evolution are therefore 

 not ontogenetically comparable for the three forms. 



Correspondingly, the number of primary plicae arising at the edge 

 of the smooth stage have also been measured at different ontogenetic 

 stages. These are more readily comparable than the height of the 

 smooth stage, however, because on many specimens of L. hellaplicata 

 (both subspecies) and a few L. lugubris the number of primary plicae 

 can be directly contrasted at the neanic-ephebic boundary. Although 

 the plicae arise early in the ontogeny of the subspecies of L. hellapli- 

 cata, they do not undergo extensive bifurcation until the early ephebic 

 stage. The number of plicae at the neanic-ephebic boundary 

 therefore generally reflects the original number of primary plicae at 

 the edge of the smooth stage. In L. lugubris the plicae normally 

 arise at a later developmental stage, but on rare specimens with a 

 small attachment scar, also arise near the neanic-ephebic boundary. 

 In these specimens, the number of primary plicae is equal to the 

 number found in specimens where the plicae arise at a later develop- 

 mental stage. This indicates that the number of plicae is not onto- 

 genetically controlled in L. lugubris, and relatively constant in number 

 regardless of where they arise on the shell. Therefore, there is 

 some validity in comparing the initial number of primary plicae on 

 L. lugubris with subspecies of L. hellaplicata, despite the variation 

 in ontogenetic stage at their first appearance. 



Important evolutionary trends noted in progressively younger 

 Turonian representatives of the Lopha lugubris lineage are : 



1. Decrease in the maximum size attained by the species (fig. 2). 



2. Slight decrease in the relative convexity of the left (lower, 

 attached) valve, and ventral migration of the high point of the valve 

 (figs. 3a, 4a, b). 



3. Gradual increase in the relative convexity of the right (upper, 

 free) valve, particularly in the umbonal region (fig. 3b). 



