218 PROF. E. EAT LANKESTER ON WALRUS-TUSKS 



cliecus Huxleyi show a somewhat deeper and more constant fluting ; whilst, further, it 

 is distinctly to be noticed that the transverse section is narrower than that of T. ros- 

 marus ; that is to say, the fossil tusks are more compressed laterally than are the living 

 ones. 



It is not possible to give the curvature of the Walrus-tusks in a numerical form ; but, 

 as compared with those of living Walrus, the fossil tusks exhibit a more marked curva- 

 ture. The tusks of living Walrus vary in this respect very greatly. Whilst the tusks 

 of females are more curved than those of males as a rule, we also find that some male 

 tusks are nearly straight, whilst others are curved. As to absolute size, there can be no 

 doubt, from the measurements which I have taken, that the fossil tusks are larger than 

 the average of recent specimens. The largest tusk in the College-of-Surgeons' Museum 

 is 26J inches long *, as measured in a straight line, between the extreme points of base 

 and crown. The fossil specimen of the tusk of a male Trichecus Huxleyi drawn in fig. 1 

 would have measured, when complete, as much ; the fragment drawn in fig. 3 indicates 

 a much larger tusk ; whilst the tusk of a female drawn in fig. 2 is larger than any recent 

 female tusk which I have examined. 



APPENDIX, July 17th. 



The varieties of form presented by the tusks of the recent and fossil Walrus are due, 

 as pointed out above, among other things, to two independent conditions, which may 

 affect any two tusks compared in different degrees, viz. attrition and growth. Growth 

 does not necessarily proceed pari passu with attrition ; and consequently tusks of the 

 same age may be of various lengths, owing to the varying amount of attrition to which 

 they have been subjected. 



The growth of the tusk consists in a continual addition to its base, that which was 

 the base or socketed portion of the tusk at one period of life becommg the naked and 

 even the terminal portion at a later period (see woodcut, where 1, 2, 3, 4 represent suc- 

 cessive bases with pulp-cavity). At a certain period of life the tusks cease to grow, as 

 is indicated by the reduction and even complete filling of the shallow pulp-cavity. The 

 biggest tusks will necessarily, ceteris paribus, be those which, having ceased to grow, 

 have during the period of growth suffered least from attrition. 



The mode of wearing-down of the tusk is made obvious from the examination of worn 

 tusks, when the new outline formed by the wearing-down of the tusk is found to have a 

 certain relation to the fluting of the surface, which, in an unworn tusk, is parallel with 

 the unworn outline. It is found that the wearing of the tusk is greater on the convex 

 or anterior border of the tusk than it is on the concave or posterior border, and is so 

 considerable on the convex face as to preserve the tapering form of the tusk although 

 the point is rapidly worn away. This is shown in the woodcut, where the successive 

 outlines, A, B, C, D, indicate a series of stages in the attrition of the tusk. If the full- 

 grown tusk had never suffered in any way from attrition, it would present the complete 



* The specimen referred to in a previous note, however, measures 31 inches in length along the chord, and 34 , : 

 inches alona; the sreater curvature. 



