2l6 PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 



sible to draw a line between them. According to v. Ihering, the chief 

 characteristics of T. santacruzensis are : 



i. It has only 8 varices. 2. The flat part of the whorls is slightly 

 ascending toward the suture (in T. patagoniciis it is said to be descend- 

 ing or excavated). 3. The form of the shell is more elongated and the 

 canal longer. 



I would make the following remarks on these three points : 



1. The number of varices increases with age, but it is already variable 

 in the young shell. Indeed, we have specimens of the same size as v. 

 Ihering's figure, which have only 8 varices ; but many others have more 

 at the same size, 9-11. The average number, in individuals a little larger 

 than v. Ihering's, is between 10 and 12, but sometimes, on the last whorl, 

 the number increases rapidly, reaching 16. Our largest individual, from 

 Darwin Station, however, has only 12. Sowerby's figure represents an 

 individual of a little more than medium size (height, 69 mm), possessing 

 very numerous varices. 



2. There is a slight variation as to the flat upper part of the whorls, it 

 being more or less ascending toward the suture, but in most cases it is 

 almost horizontal. In no case, even in individuals corresponding closely 

 to Sowerby's figure, it is descending toward the suture. The excavated 

 appearance is due to the strongly elevated varices. 



3. The external form is very variable. As a rule, younger individuals 

 are more slender, older ones comparatively broader, but there are many 

 exceptions to this rule in young ones. Our largest individuals, however, 

 are all short and broad. 



With the external form the length of the canal varies, and there is 

 considerable variation as to the size of the umbilicus and the develop- 

 ment of the spiral striae, which in many individuals are entirely absent, 

 and in a few very strongly developed. They are best developed in com- 

 paratively young specimens. 



This species differs from T. laciniatus in the following points: i. The 

 upper flat part of the whorls is not crossed by the varices, and this flat 

 part is broader. 2. The lobe formed by each varix on the angulation is 

 more strongly developed, more acuminate and recurved. 



In the occasional presence of spiral striae this species approaches also 

 the recent T. geversiamis (Pall.) (see : Kuester & Kobelt, 1878, p. 275, 

 pi. 72, f. 13, pi. 73, f. i), which is probably nothing but a variety of 

 T. laciniatus. 



