loo: 



NATURAL SCIENCES OF PHILADELPHIA. 



235 



the latter part of whorl 2, but the ribbing above the shoulder grows 

 weaker. 



Whorls 5, G. — Characterized by the cancellated stage, but toward the 

 entl of 6 the ribbing above the shoulder is gone and the suture tuber- 

 cles are weak. The shoulder tubercle strengthens and the ribbing 

 below it is well developed. Whorls covered with fine spirals from the 

 shoulder downward, but they are stronger below on the branchial 

 siphon. 



Whorl 7. — Still characterized by the cancellated stage. The suture 

 tubercle and the spirals immediately below the shoulder both disappear. 

 The spirals are well developed, however, on the branchial siphon. 



Fig. 1. 



Fig. 2. 



Fig. 1. — Apex of Athleta dayi. St. j\Iauricc, Louisiana. In the second whorl the 

 short and ahiiost suppressed curved ril) stage is seen passing into the tui^er- 

 cled ribs of the cancellated stage. Length about 1.2 mm. 



Fig. 2. — Athleta dayi. St. Maurice, Louisiana. Mature individual exliibiting in 

 its last whorl the features of the spiny stage. In the mature sculpture it 

 is practically indistinguishable from the adult specimens of A. petrosa. 

 Length = 27.5 mm. 



Toward the end of the whorl the ribbing below the shoulder disappears 

 and the shoulder tubercles become spine-like, so that the cancellated 

 stage ends with the close of whorl 7. 



Whorl 8. — Characterized by the spiny stage. Shoulder spines 

 sharp. Spirals on the branchial siphon only. Shelly smoothing of 

 the preceding whorl is slight. 



Remarks. — This form is distinguished from the St. iMaurice race of 



