78 Studies in Geology, No. 3 



mm. Breite an der Miindung 27 mm., oben 15 mm." — 

 Grzybowski, 1899. 



The variety rotiindata differs from the true nelsoni only in 

 having the whorls evenly rounded, with the carina absent or 

 very faintly noticeable. The sculpture is exactly similar to 

 that of nelsoni. The point of greatest whorl-diameter is 

 at the position of the carina, but the surface is evenly 

 rounded. Dimensions similar to those of nelsoni. 



This variety is closely similar to T. terehralis var. turri- 

 tissima Sacco,"^ from the Aquitanian and higher horizonci 

 (chiefly Helvetien) of Italy and central Europe, but the 

 whorls are more convex and the sculpture apparently finer 

 and more diverse than in turritissima. 



Upper Zorritos. Quebrada del Toro. 



Turritella nelsoni var. tnillissatia n. var. 

 Plate III— Fig. 8. 



Shell large, heavy. Whorls angular, roughly sculptured 

 with strong, cordate threads. Surface of whorls monangular 

 in profile, a prominent carina at about the anterior fourth 

 and the surfaces before and behind the carina plane in 

 profile. Sculpture of spiral threads clearly differentiable into 

 four orders of strength ; of these the first is represented by 

 one, the carinal thread, which is very strong and cordate ; 

 behind the carina are four threads of the second order, which 

 are also strong; between the second and third of these is a 

 thread of the third order which occurs constantly in all speci- 

 mens seen, invariably located nearer to the third secondary 

 thread behind the carina: other tertiary threads occur be- 

 tween the first and the second and between the fourth and 

 the suture, but they are not constant and are always slightly 

 weaker than the first mentioned. Before the carina is one 

 secondary thread half-way between it and the suture, with a 

 tertiary thread on each side of it, and a basal secondary 



•"Idem., fig. 27. 



