88 PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 



(5 or 6), while even in smaller individuals (Nos. 12 and 13) a very high 

 number (7 or 8) is present. 



Thus is seems that the number of sulci is very variable : at a length of 

 ca. 50 mm. it is about 3, increasing at ca. 70 mm. to 4 and 6, and at over 

 80 mm to 7 and 8, and even more ; but sometimes there are specimens, 

 in which this number does not correspond to this rule : Sowerby's figure 

 23 shows an individual 47 mm in length, that has 5 or 6 sulci, where we 

 should expect only 2 or 3, and v. Ihering's individual of C. alia, of 45 

 mm length, has 9, where we should expect not more than 3. Although 

 this latter individual seems at the first glance to differ strikingly from 

 all our individuals, Sowerby's figure 23 and our specimen No. 9 form the 

 transition between both extremes. 



Aside from the number of sulci on the area, v. Ihering (in 1899) gives 

 the following differences between C. alta and dalli: (i) C. dalli is the 

 larger species; (2) the apex in C. alta is more anterior; (3) the surface 

 ornamentation differs in both. 



Characters (i) and (2) are of no value, since there is in size and shape 

 considerable variety, as is shown in our series : indeed, one of the largest 

 specimens (No. 13) shows the apex more anterior than any of the rest, 

 and this one agrees in all other respects with v. Ihering's C. dalli. Further, 

 it will be remarked that our No. 9, which, according to the sulci, is the 

 one that approaches C. alta of v. Ihering most closely among our indi- 

 viduals, shows an almost central apex, a character of C. dalli, and the 

 same is true of No. 12. 



As to character (3), v. Ihering says that C. alta has in the middle of the 

 shell ca. 30 radiating ribs, as broad as the intervals, but that these ribs 

 are wanting on the anterior and posterior parts of the shell. In C. dalli, 

 however, he says that these ribs are found all over the surface. (In 1897 

 he described in C. dalli the surface-ornaments as striae rather than ribs.) 



It seems to me that he has not seen the original surface of the shell. 

 In our specimens (see fig. 4) where the original surface is preserved 

 it is rugose ("subrugosa" Sowerby), /. e., sculptured by very fine radiating 

 striae or impressed lines, which are crowded, but some of them at certain 

 intervals (3-10 mm) are a little more distinct (deeper). The intervals 

 between the striae are flat and crossed by fine undulating lines of growth : 

 thus there is nothing that might be called "ribs." But when the outer 

 surface of the shell is destroyed by fossilization, the condition described 



