12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I44 



there are no tetrabasal systems in adulte of any post-Cretaceous 

 species. On chart i, all those genera having monobasal apical systems 

 are above the bold dashed line, and all those with tetrabasal systems 

 below. 



It is not possible to tell whether or not this change to a monobasal 

 system was caused by fusion of the genital plates, resorption, or 

 nondevelopment of genitals i, 3, 4, 5. There is no trend toward 

 increase in the size of genital plate 2 and no corresponding decrease 

 in the size of the other genitals. In one of the earliest cassiduloids, 

 Clypeus agassisi (text fig. 13) genital 2 is just as large, and the 

 other genitals as small as in later species such as Petalohrissus seti- 

 fcnsis Cotteau from the Senonian. Gordon (1929, p. 310) found in 

 the Recent species Echinarachnius parma Lamarck that only one 

 genital plate ever formed, with the other three present only in the 

 form of remnants of larval spicules. Hyman (1955, p. 533) says 

 that "in the Cassidulidae the four genital plates are fused with the 

 madreporite," but she gives no evidence for this statement. 



This change from a tetrabasal to a monobasal apical system is 

 remarkable not only because of its parallelism in all the lineages of 

 the cassiduloids that extend into the Tertiary, but also because of the 

 abruptness with which it occurs. The lack of any trend toward a 

 reduction in the size of the other genital plates, and the absence of 

 any specimen with a monobasal apical system before the Senonian, 

 suggests that this change was produced by parallel mutations and 

 parallel selection. It might be suggested that the reason for the 

 change is that a monobasal apical system is structurally stronger 

 because it lacks the sutural area found in a tetrabasal system. How- 

 ever, almost all pre-Senonian cassiduloid specimens have their apical 

 systems preserved intact. It is, therefore, difficult to understand why 

 a tetrabasal apical system would simultaneously become such a lethal 

 character for all cassiduloids. Perhaps the mutations that produced 

 the monobasal system were linked with another feature of greater 

 selective value. 



BOURRELETS 



Bourrelets are present in the earliest cassiduloids, in which they 

 are slightly to moderately developed, have vertical sides, and bulge 

 slightly into the peristome (e.g., Clypeus). By Cenomanian time the 

 bourrelets are often more prominent, as in Catopygus or Ochetes. 

 They reach the zenith of their development in the Senonian, particu- 

 larly in the Maestrichtian, when they become very large in such 



