STROBILOPS, CANADA AND UNITED STATES. 19 



interparietal lamella is developed deep within. Internal bar- 

 rier consisting of a columellar lamella, several basal folds and 

 often other palatal folds above the periphery. Type, S. lahy- 

 rintJiica. 



All living American species except 8. hubbardi belong to 

 this group. Also the European Tertiary Gruppe der Stro- 

 bilops costata of Wenz (1915, p. 85). In the latter the knots 

 of the lamellge appear to be very weak or wanting; "von 

 einer Zahnelung der Lamellen habe ich nichts bemerkt, " 

 Wenz observes ; but in the Pliocene ^S^. romani (PI. 12, figs. 11, 

 12, 13) roughened nodes are present though inconspicuous. 



There are two groups of species : first, that of S. Idby- 

 rinthica, iu which there are numerous basal folds and one or 

 two above the periphery; this group including also S. texas- 

 iana and 8. affinis. Second, the group of 8. strebeli, in which 

 there are three (or in 8. cenea, four) basal folds and none 

 above the periphery ; the Mexican and South American species 

 belong here. Both of these groups appear to be represented 

 in the Tertiary fauna of Europe. 



8pecies of Canada and the United 8tates. 



1. Trochiform or dome-shaped species, in which the height 



much exceeds half the diameter; umbilicus narrow or 

 moderate, contained 6 to 12 times in the diameter. (2) 

 Subdiscoidal species, the height about half of the diameter ; 

 umbilicus contained between 3 and 4 times in diameter. 



^S'. hubbardi, No. 15. 



2. Spire convexly conic or dome-shaped; elevated; usually 5 



or more basal and palatal folds. (3) 

 Spire rather low conic, its outlines but slightly convex ; less 

 elevated; periphery angular; 3 or 4 basal folds, none 

 above the periphery. 8. cenea, No. 4. 



3. Diam. 2.3-2.5 mm.; basal folds strongly unequal. (4) 

 Diam. 2.75-2.9 mm.; basal folds short, subequal, disposed 



in a regular curve. 8. affinis, No. 3. 



4. Finely ribbed. 8. laiyrinthica, No. 1. 

 Coarsely ribbed ; southern. 8. texasiatia, No. 2. 



