PALEONTOLOGY ZORRITOS FORMATION 77 



appearance of adults, and occur in fairly uniform size, 

 without enough specimens clearly transitional to the large 

 ones to make their identity certain. In controversy to that is 

 the fact that the development of the sculpture is identical 

 for the two the small forms show exactly what one would 

 expect to find in the young of nehoni, and it does not seem 

 rational to separate them. 



The small forms are strikingly similar in many ways to 

 T. plebeia Say, 67 of the Miocene of Maryland and neighbor- 

 ing regions. The neanic whorls of plebeia are subcentrally 

 bicarinate, apparently banded as in the young of nelsoni. 

 The sculpture is generally similar but in plebeia the diversi- 

 fication of the threading appears in earlier whorls than in 

 nelsoni, and the sharpness of the threads is greater. In addi- 

 tion, the sutural depression in plebeia is deeper. The adult 

 forms are very close to T. terebralis var. subgradata Sacco, 68 

 from the Helvetien of Italy, differing chiefly in having less 

 coarse and more diversely threaded sculpture. 



Upper Zorritos. Quebradas del Tore and los Angeles. 



Turrit ella nelsoni var. rotundata (Grzybowski) 

 PLATE III Fig. 7. 



Turritella suturalis Nelson (ex parte) Trans. Conn. Acad., vol. 2, 

 p. 188, 1870; not of Sowerby, nor of Phillips. 



Turritella rotundata Grzybowski, Neues Jahrb. f. Min., etc., Beil. 

 Bd. 12, p. 643, pi. 20, fig. 6, 1899. 



"Die Umgange sind stark convex, gerundet, die Naht tief. 

 Das Gewinde ist mit Spiralstreifen verziert, von denen die 

 4 untersten zusam>men 1/3 der Umgangshohe einnehmen, 

 schwach angedeutet sind, und nahe beieinander liegen. Die 

 oberen 4-5 werden zu starkeren Spiral-rippen und es schalten 

 sich zwischen sie noch ganz feine secundare Streifchen ein. 

 xxx Hohe des Bruchstiickes von 4 unteren Ungangen 65 



6T Md. Geol . Surv., Miocene, p. 234, pi . 56, figs. 4-9, incl., 1904. 

 68 Piemonte, pte. 19, p. 9, pi. i, fig. 26, 1895. 



