INSTITUTE OF SCIENCE, PHILADELPHIA. 315 



It is not improbable that a complete collection of Vicksburg Eocene fos- 

 sils, which is still a desideratum, would contain a precursor of the old Mio- 

 cene form of this species, but the oldest material available for my present 

 study is that from the Chipola beds. Here we have a form which is not dis- 

 tinguishable by any marked characters from the recent one described by the 

 writer in 1889 (Rep. Blake Gastr., p. 264) as T. {Torada) acropora. It is a 

 slender shell covered all over with fine spiral grooving, which is peculiarly 

 narrow and sharp, so that the interspaces form even, narrow, regular, elevated, 

 close-set threads. The nucleus is smooth, obliquely set, minute and loosely 

 coiled. The variations are few. The normal form has an equatorial larger 

 thread which slightly angulates the whorl, and the spiral threads have a tend- 

 ency to alternate in size, though the difference in size is slight. Under the 

 glass here and there wider threads are seen among the others, and in the living 

 acropora these wide ones are articulated with brown markings. The varia- 

 tions are — (i) the shell differs somewhat in slenderness, some being a little 

 stouter than others ; (2) the width of the spiral grooving varies, some having the 

 grooves wider than others ; (3) the tendency to alternation in size of the spiral 

 threads is more or less pronounced, the threads being perfectly even and simi- 

 lar in some and regularly alternate in others, with all intermediate gradations ; 

 (4) in addition to the equatorial carina another is developed just behind the 

 suture, or the equatorial carina becomes so feeble as not to interrupt the 

 rounded outline of the whorl. 



These variations are characteristic of the Miocene and recent shells, and 

 the same types are found in the Pliocene also, but with many others. The 

 variety acropora is figured from a Pliocene specimen on plate 16, as figure 4. 



In the Pliocene the conditions of existence for the Turritellas seem to 

 have been particularly favorable. In this species one evidence of this is the 

 large size to which many of the specimens attain, and the perfection of their 

 ornamentation. 



The variety acropora appears as before, and is not rare. It shows the 

 mutations above described and others. The latter may be specified as fol- 

 lows : (i) The number of wide spirals in proportion to the fine ones increases 

 especially between the suture and the equator of the whorl in front of it ; 

 (2) the spirals, while increasing but little in size, become more distant and the 

 equatorial carina less pronounced ; this variety, which may be called var. 

 Biirnsii, has a coarser look and more rounded whorls than the type ; (3) slight 

 transverse waves cross the whorls, becoming most evident at the periphery 

 and undulating the equatorial carina; all gradations in this respect may be 

 observed ; the combination which formed Prof. Heilprin's type, and which 

 may be called var. subanmilata, has the sutural and equatorial carina strong, 

 wide spirals numerous behind the equator, narrow spirals elsewhere rather 

 .feeble, the undulations of the equator well marked and extending backward 

 nearly to the suture ; the shell rather stout, measuring 27 by 7 mm. This 



