VESTINAUTILUS CRASSIMARGINATUS. 81 



the thickened rim; here it is 4 mm. thick; below the rim, that is upon the 

 umbilical declivity, it is 2 mm. thick; this thickness reaching about halfway to 

 the line of inclusion of the preceding whorl, in which region it is reduced to 

 1*5 mm. 



Affinities. — I am not acquainted with any species with which this may be 

 strictly compared, though the young shell, in which the coarse and heavy rim of 

 the umbilical border is undeveloped, is not unlike that of Fcstinantilus cariniferiis, 

 J. de C. Sow., and was mistaken by me for that species until I had examined the 

 specimen more closely and had seen the characteristic ornamentation of fine 

 transverse lines. Vestinautiliis crassimarginatns differs from V. carioiiferus in 

 having much more slowly increasing whorls, and in the presence of the very 

 strong and coarse rim bordering the umbilicus, besides the fine and regular 

 transverse strige covering the whole of the test. 



Beviarks. — The question of the generic affinities of this shell is a difficult one 

 to settle in the absence of a sufficient number of specimens wherewith to study 

 the stages of growth from the very young to the old shell, and their modifica- 

 tions. The slow increase in the diameter of the whorls, their slight inclusion, 

 and the great width of the umbilicus resulting from these two conditions, gives 

 the shell, at first sight, a very different aspect from that of the more typical 

 members of the genus to which I have referred it. Nevertheless, after careful 

 study, I consider that these differences are outweighed by the resemblances, which 

 consist of (1) the crater-like shape of the umbilicus, (2) the general form of the 

 whorls, and (3) the non-tuberculated umbilical rim. It is unfortunate that the 

 septation is not to be got at, as this would have aided very much in the determi- 

 nation of the affinities of the fossil. 



Dimensions. — The largest specimen measures 127 mm. in its greatest diameter, 

 and 102 mm. in its smallest. The specimen is thus elliptic : and that this form is 

 the result of distortion by rock pressure is proved (1) by the asymmetry of the 

 whorls both inner and outer, which do not follow the same spiral throughout their 

 course, and (2) by the fact that in a young specimen there is no asymmetry at 

 all. The height of the whorls relative to their width is as follows : — height, from 

 centre of area of inclusion to centre of ventral area, 24 mm. ; width, from the 

 summit of one umbilical rim to the summit of the other, 44 mm. ; thus the ratio 

 of height to width is as 6 : 11. 



I am indebted to the kindness of Prof. Grenville Cole for the loan of a 

 specimen (the young one mentioned above) contained in the museum of the Royal 

 College of Science for Ireland, Dublin, and from the same locality as the two 

 full-grown specimens. 



Locality. — Little Island, near Cork. 



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