84 STUDIES IN GEOLOGY, No. 3 



eines aus 6 Windangen bestehenden Bruchstiickes 37 mm., 

 Breite, unten 12 mm., oben, 4 mm." Grzybowski, 1899. 



In addition to the seven subequal threads noted by Grzy- 

 bowski there is an occasional microscopically visible inter- 

 stitial thread on some specimens, particularly between the 

 third and fourth primary threads. Grzybowski emphasized 

 the weakened sixth rib in his description, but his figure does 

 not correspond in that it shows that rib to be equally as 

 prominent as all save the fifth and seventh, and there is 

 thus question as to the validity of his statement. The speci- 

 mens here studied agree with his figure, and it is undoubtedly 

 true that his mention of a weakened sixth rib is an error. 



This form is here treated as a variety of prenuncia in order 

 to preserve the rationality of relationships within the group 

 of Turritella nelsoni. To make prenuncia the variety of 

 inconspicua would reverse the order of varietal relationships 

 in the group, and a desire to avoid inconsistency as well as 

 to preserve the meaning of names has led to the present 

 proposition. Thus placed, T. prenuncia and its variety 

 inconspicua parallel the other two pairs (nefconi-var. rotun- 

 data : infracarinata-var. zorritensis) which comprise carinate 

 and rounded forms with similar sculpture. 



Zorritos formation. Zorritos. 



Turritella robust Grzybowski 

 PLATE IV Fig. 5. 



Turritella sp. ind. Nelson, Trans. Conn. Acad., vol. 2, p. 190, 

 1870. 



TurriteVa (Haustatnr) robusta Grzybowski^ Neues Jahrb. f. 

 Min. Beil. Bd. 12, p. 646, pi. 20, fig. 3, 1899. 



" Whorls broad and very concave; sutures indistinct. 

 Surface just above each suture marked by a very strong 

 ridge. Intermediate surface marked by a few distinct con- 

 centric lines, five to seven on each whorl." Nelson, 1870. 



