ORTHAULAX, A TERTIARY GUIDE FOSSIL. 



By C. Wythe Cooke. 



INTRODUCTION. 



Since the publication of Dall's "Tei'tiary 

 fauna of Florida" in 1890,' when the genus 

 Orthaulnx was for the first time adequately 

 described, much im{)ortance has attached to 

 Orthaulax as a horizon marker. Twenty-five 

 years later Dall - voiced current opinion when 

 he said : 



This genijs is the most characteristic and tj-pical of those 

 belonging to the middle < )ligofeue of our southern Coastal 

 Plain and the Antilles, including Middle America. It 

 does not appear in the Vickslnirgian fauna or the nummu- 

 litic Ocala beds of Florida ; it seems to have become 

 extinct before the development of the Oak Grove, Fla., 

 fauna. So far it has been recognized in the middle Oligo- 

 cene of Santo Dondngo, Cuba, Antigua, the Canal Zone 

 of Panama, the Tampa silex beds, the Oligocene of Rain- 

 bridge, Cia., and the lower bed at Alum Bluff, with its 

 stratigraphically e<iuivalent marl of the Chipola River, 

 Fla. It is not known from the Howden beds of Jamaica, 

 which are doubtless younger than the Haitian Oligocene 

 explored by CJabb, if indeed the latter be not divisible 

 into several distinct horizons. 



But the range in time appears so narrow and the genus 

 so sharply characterized that, according to our present 

 knowledge, the discovery of a species of Orthaulax in a 

 Tertiary fauna may be taken as positive proof of'its middle 

 Oligocene age. 



Since that statement was written the arbi- 

 trary boundary between the Miocene and the 

 Oligocene has been shifted downward, so that 

 the range in time of Orthaulax as then known 

 straddles the greater part of the Oligocene and 

 the lower Miocene of the present standard 

 geologic time scale. The recent rediscovery of 

 the genotype in an unsuspected stratigraphic 

 position in Santo Domingo, as well as the 

 attempt to identify anotlier species from Santo 

 Domingo, made necessary a critical study of 

 all the available specimens of the genus. The 

 facts assembled in this itivestigation appear to 



1 Wagner Free Inst. Sci. Trans., vol. 3. 



2 Dall, W. H., A monograph of the moUusoan fauiia of the Orthaulax 

 pugnai zone of the Oligocene of Tampa, Fla.: U. S. Nat. Mus. Bull. 90, 

 p. 86, 1915, 



me so interesting and imjjortant tliat it seems 

 worth while to restate tlie old ami to put the 

 new on record. 



THE GENIIS. 



Orfliaiihix is a marine stromboid gastropod 

 resembling the common conch in many respects 

 but curiously difl'erent from it in others. While 

 still comparatively young the animal extends 

 the outer lip of its shell to the tip of the spire, 

 and continued growth envelops the entire spire 

 in the domelike body wliorl. The narrow space 

 between the spire and the enveloping whorl soon 

 fills with enamel. Some strombs exhibit a 

 similar tendency to cover their spire with 

 enamel and to carry the lij) to the summit, but 

 I know of none in which the process begins so 

 j'oung or proceeds so far as in Orthaulax. 

 Uippochrenes also extends its lip to the spire, 

 but this does not occur until the animal has 

 attained maturity, so that its spire is not wholly 

 involute as in Orfhauh.r. Figure 1, Plat(> II, 

 representuig a living species of Strombus, has 

 been introduced for comparison. In all the 

 known species of Orthaulax the outer lip lacks 

 the prongs antl knobs that characterize many 

 species of Strotiihus, Rostellaria, and related 

 genera. The Eocene genus Calyptraphorus 

 bears a superficial resemlilance to Orthaulax, 

 but the covering of the spire of Calyptraphorus 

 seems to be simply enamel spread over it after 

 the animal attains maturity and is not an inte- 

 gral part of the shell. 



The genus Orthaulax was defined in 1872 by 

 Gabb, who used as genotype Orthaulax inortia- 

 tus, from Santo Domingo. In 1887 Heilprin 

 described a new species, pugiiax, from the 

 "silex bed" of Tampa for which he proposed 

 the new genus Wagneria. Tlu-ee years later 

 Dall described a third species, O. gahhi, from the 

 Chipola marl of Florida, and Maury has recently 

 added a fourth species, 0. agvadillensis, from 



