STROBILOPS. 17 



ably, and certainly from Lower Oligocene to mid-Pliocene 

 times. 



3. Group of 8. uniplicata, for S. unipUcata and its sub- 

 species. Upper Oligocene to Upper Miocene. 



A few of the Tertiary species are not known fully enough 

 to decide upon the group. Nearly all of the living species 

 appear to be referable to one or other of Wenz's three groups, 

 though the definitions given by him require some modification 

 to provide for them, as would be expected. 



The number and relative size of the basal folds can hardly 

 be considered a subgeneric character, being rather variable 

 among closely allied species. Probably forms with a columel- 

 lar lamella and three basal folds, with none above the peri- 

 phery, represent a rather primitive stage, this number being 

 found in various Tertiary species as well as in the peripheral 

 South American forms. At the same time, some early forms, 

 such as 8. headojiensis and siihlahyrinthica possessed more 

 numerous basal folds and also some above the periphery (pala- 

 tals), just as in the Recent 8. lahyrinthica. It appears that 

 there has been some parallel evolution in these folds, and an 

 increased number may appear in different phyletic lines, if 

 the multilaminate forms are actually the later products of 

 evolution. 



The interparietal lamella may be either present or wanting 

 in the group of 8. hub'hardi, but it is certainly absent in most 

 of the fijiely striate species (group 1 of 'Wenz=: Eostrol}ilops), 

 and it is present in the costate ones (group 2 = typical 8tro- 

 hilops). Probably this lamella was present in the ancestral 

 stock, but degenerated in some groups. Thus, it is developed 

 in 8. unipUcata, but very weak or usually absent in the modem 

 8. huhhardi. The degree of minute sculpture of the edges of 

 the parietal lamellaa varies rather widely ; Wenz did not find 

 prickly knots on the lamella of his group 2, but they are 

 present, though inconspicuous, in the Pliocene ;S^. romani (pi. 

 12, figs. 11, 12, 13), and are more or less conspicuously de- 

 veloped in all living species of the same group. 



The following arrangement is here adopted : 



