ART. 21 TERTIARY MOLLUSKS FROM FLORIDA MANSFIELD 7 



are more expanded at the anterior end than posterior, and wind 

 against the base of the preceding whorl. Earliest li^ turns smooth 

 and inflated; the following 4 to 5 turns are medially carinate; and 

 the following turns are marked by two spiral carinae, which are 

 more prominent on the earlier than later whorls. The lower carina 

 marginates the precipitous base and gradually increases in strength 

 in ascending the whorl. Aside from the carinae, there are 12 to 

 16 moderately strong, subrounded primary spiral threads on each 

 whorl, which are more closely spaced on the early whorls than on 

 the later whorls. Two to three threadlets intercalate the primary 

 spirals. Aperture subovate in outline. 



Dimensions. — Holotype (U. S. N. M. No. 371335) measures : Alti- 

 tude, 66 mm, ; greatest diameter, 21 mm. 



Type locality. — Tamiami Trail, 42 miles west of Miami, Fla. 



Some of the broken specimens represent larger shells than the 

 holotype; one of these measures 33 mm. in diameter. 



The new species is named for Gerald M. Ponton, of the Florida 

 Geological Survey. 



It is nearly related to Tunitella gatunensis Toula but differs from 

 the latter in having a greater apical angle and weaker and more 

 rounded spirals on the posterior slope. 



Turritella alcida Dall, a species occurring in the Oak Grove sand 

 of the Alum Bluff group, is similar in outline and in sculpture to 

 T. pontoni but differs from it in having wider-spaced carinae and 

 more closely spaced spirals. 



TURRITELLA COOKEI GLADEENSIS, new subspecies 



Plate 2, Figubes 1, 2, 3 



Description. — Shell rather large, acute, the apical angle being 

 about 18°, and consists of 14 whorls on the broken holotype, prob- 

 ably originally about 20 whorls. Suture grooved but not deeply 

 impressed. Anterior one to two whorls on some specimens rounded in 

 outline and much more expanded than preceding. Apical whorl 

 rather small, smooth, and nearly rounded in outline ; following 8 to 

 10 whorls medially depressed and ornamented mainly by two crenu- 

 lated spirals, the anterior being the stronger and first to originate. 

 On the later whorls there are 6 spirals. In ascending the whorl, the 

 first and third spirals are weak and about equal in strength; the 

 second and fourth are a little stronger ; the fifth is the strongest spiral 

 and forms the periphery of the whorl ; and the basal spiral is weaker 

 than the preceding and lies closely against the suture. All the spirals 

 except the basal one are crenulated. The surface of the shell is 

 marked by spiral threadlets, which overrun the spirals and interspaces. 



