Paleontology Zorritos Formation 45 



Family BLXCIXIDAE 



Genus Solexosteira Dall 



Solcnostcira altcrnata (Xelson) 



Plate I — Figs. 10, 11. 



Cuma altcrnata Xelson, Trans. Conn. Acad., vol. 2, p. 198, pi. 7. 

 figs. 3, 4, 1870. 



Shell of good size, fairly thick, well proportioned. Spire 

 conic, acuminate, turreted, with about 25/^ smooth embryonic 

 whorls, the following whorls rather sharply convex, and 

 sculptured with large longitudinal folds, the intervals wider, 

 of which there are eight or nine on the body whorl, and 

 sharply defined, closelv spaced spiral threads. The folds 

 rise into points at the keel of each whorl, and on the 

 shoulder of the body whorl the points are sharp in hori- 

 zontal direction. The spiral threads may be separated into 

 three orders : of the first order there are about 28 on the 

 body whorl between the suture and the base of the outer 

 lip, with several more on the basal fascicle ; between the 

 primary threads are varying numbers of secondaries — near 

 the sutures there are as many as four or five, whereas mesi- 

 ally one is the rule. Tertiary threads are interstitial between 

 the secondaries throughout. The dififerentiation of the three 

 orders of threads is sharper above and below the keel than 

 it is mesially. Fine growth-lines cover the entire shell, cross- 

 ing the spiral sculpture. The body whorl is concave below 

 the keel, and expanded into a curving ridge about the 

 umbilicus, which is distinct but shallow, and funnel-shaped. 

 The anterior canal is open, deep and reflexed. The pillar is 

 simple, with a thin callus. The aperture is oval in shape, 

 dififerentiated from the canal ; the throat is lirate, the lirae 

 being small and equi-distant. The basal fasciole is strong. 

 Height, 52; diameter. 33 mm. 



This species is a beautifully sculptured, gracefully pro- 

 portioned shell, perhaps the most pleasing 10 look upon of 



