Paleontology Zorritos Formation 83 



Aside from the difference in development of the sculp- 

 ture, there is the difference in size noted, with slight differ- 

 ence in proportion. The adult whorls are verv constantly 

 similar in sculpture, bearing five primary threads with two 

 secondaries, one on each side of the posterior primary. The 

 species is apparently normally found small in the Lower 

 Zorritos formation, but somewhat larger forms, stratigraph- 

 ically not allocated, occur in Nelson's collection. Length of 

 large fragment bearing 5 whorls, 34 mm. ; maximum diam- 

 eter, 12 mm.; minimum diameter, 5 mm. 



The detailed discrimination between the neanically differ- 

 ent forms, of this species is offered in consideration of the 

 normal biologic importance of ontogenetic development as a 

 key to identity. Were anything to be gained by it, the two 

 might be considered as separate, but the identity of the 

 adult whorls detracts from the value of a differentiation, if 

 only because by. far the most collected material shows only 

 the later whorls of the spire, the more fragile neanic whorls 

 having usually been broken off. These specimens which 

 have more of the spire preserved show that there is appar- 

 ently a predominance of the neanic type (a). The sculpture 

 of the adult whorls is the same in ground plan as that of the 

 other members of the nelsoni group, and in number of rib? 

 it is entirely similar to that of the much larger T. infracar- 

 inata. T. prcnuncia is undoubtedly the diminutive fore- 

 runner of infracarinata. 



Lower Zorritos. Quebrada Zapotal. 



Turritella prcnuncia var. inconspicua (Grzybowski) 



Plate IV — Fig. 4. 



Turritella inconst>icua Grzybowski, Neues Jahrb. f. Min., etc., 

 Beil. Bd. 12, p. 644, pi. 20, fig. 4, 1899. 



"Umgange gewolbt, mit 7 gleichen Spiralleisten, von 

 denen die sechste von unten die schwachste ist und wie ein- 

 geschaltet zwischen die Nachbarleisten erscheint. Hohe 



