Paleontology Zorritos Formation 75 



the suture. On the fourth whorl this subsidiary thread is 

 stronger, and a distinct intermediary thread develops on the 

 band, which is gradually growing less prominent ; anterior to 

 the band another thread develops. On the fifth whorl two 

 additional threads appear above and below the one before 

 the band, making three subequal threads between it and the 

 suture. On the sixth whorl a subsidiary thread appears 

 between the posterior thread of the band and the one behind, 

 and on the seventh whorl another subsidiary thread appears 

 between this posteriormost prominent thread and the suture. 

 This sculpture continues on the spire, the threads gaining 

 strength ; on the body whorl the threading seems somewhat 

 subdued. At about the sixth whorl of the spire the band 

 has lost prominence, and succeeding whorls are rounded in 

 outline; the two threads which form the band in the neanic 

 whorls are so prominent as to give a bicarinate appearance, 

 but this is lost in later whorls. Some specimens show whorls 

 with somewhat flattened sides, but most are evenly convex. 

 Length of young specimen. 17.25 mm. ; diameter, 5.5. 



Adult shell strong, large. Whorls twelve or more, the 

 sutures distinct ; shape of whorls convex, a prominent carina 

 at about the anterior fourth of the whorl-length marking 

 the point of greatest diameter. Above the carina the surface 

 is slightly convex, with a distinct indentation just before 

 the suture ; before the carina the surface is flat to slightly 

 rounded (in cross-section), turning in somewhat sharply 

 to meet the suture. Sculpture of four primary cordate 

 threads behind the carina, the second from the carina fairly 

 constantly the most prominent, the others subequal ; between 

 the primary threads are variably placed secondaries, of which 

 that just above the carina is usually most prominent ; and 

 tertiary threads occur in most specimens, particularlv near 

 the carina. The carina may or may not be marked bv a 

 thread ; sometimes it is set ofif by two closely set secondary 

 threads. Before the carina are three to four secondary 

 threads, between which in the adult whorls tertiary threads 



