220 PROF. E. RAY LANKESTER ON WALRUS-TUSKS 



less lateral compression, greater or less amount of fluting, we have set forth the whole 

 series of elements which are factors in the form of any given Walrus-tusk. 



The effect of a greater primary curvature on the appearance of the tusk at different 

 stages of growth and attrition will he obvious from the second woodcut, one remarkable 

 fact being that a short tapering tusk may be produced at an early stage of growth 

 and attrition, not unlike that which is the final stage (outline 4 D in fig. 1) in the 

 straighter tusk. 



The causes of attrition of the Walrus-tusk are not found in the mere rubbing of the 

 points upon ice studded with sand particles, but in the longitudinal movement of the 

 tusks in digging up the sea-bottom when the Walrus is in search of Mollusca, and 

 very probably also in the longitudinal movement of the tusks against rock-surfaces when 

 they are employed to scrape off attached Mollusca, such as Limpets. 



That the tusk-tissues are very easily worn dowu is proved by the condition of a skull 

 in the collection in my charge at University College, London, where the points of the 

 two tusks in a skull of Walrus have been worn flat by movement on the shelves and 

 tables of the Museum. 



An important fact in this connexion is the nature of the sea-bottom and sea-shore 

 which were frequented by the fossil Walrus of the Crag. It is an established conclusion 

 that this late Miocene (Diestien) sea, with its Pyrula, Voluta, Cassidarla, Pholadomya, 

 and such forms, and its Teuthopbagous Whales (Zrpkioids) and its huge Sharks, was 

 not an ice-bound sea. The tusks of the Walrus, then, are only secondarily, and not 

 primarily related to its movements upon shore-ice. 



At the same time, from what we know of the structure of this part of Eurojoe, it is 

 certain that the Diestien Walrus had no very hard rocks against which to wear down its 

 tusks. ^ 



And accordingly we find (as it seems to me) a less extensive attrition of the fossil than 

 of recent Walrus-tusks, and, as I have pointed out above, a greater primitive curvature 

 and a greater lateral compression. The combined effect of these special factors in the 

 case of the fossil tusks accounts for the differences of form which some of them present 

 when compared with tusks of the Living Walrus. 



