506 Geological Society. 



marked, but semi-transparent. The fibres of the under tusk slightly 

 undulate, and present occasionally an appearance of thorny projec- 

 tions. The compartments of the fibres are easily seen, and are irre- 

 gular in size, but rounded. 



Tetracaulodon Tapiroides. — The tusks consist also of only crusta 

 petrosa and ivory, and the resemblance in the microscopic structure 

 of this species with that of T. Kochii is great. The thickness of the 

 crusta petrosa is considerable. The very irregularly-shaped corpus- 

 cules, placed at intervals of two or three corpuscular diameters, are 

 semi-transparent, and without radiating fibres in the external half j 

 but those situated in the internal half are of the usual opacity, and 

 give off numerous radiating fibres. Transverse, irregularly beaded, 

 independent fibres traverse the substance, making one distinct curve 

 in their passage across it. The cells of the interspaces are slightly 

 visible. 



The ivory is so translucent and homogeneous as to exhibit generally 

 very little character. The fibres undulate but do not divide, forming 

 an abrupt line of junction with the crusta petrosa. The form of the 

 beaded compartments of the fibre is oblong, not rounded, as in T. 

 Kochii, and they do not exhibit thorny projections. These are the 

 only marked differences in the two species. 



The cells of the semi-transparent interfibral space are generally 

 visible. 



Missourium. — The constituents of the tusks are likewise crusta pe- 

 trosa and ivory j but their intimate structure, Mr. Nasmyth says, is 

 more peculiar, so far as his examination has extended, than that of 

 the tusks of the preceding animals. 



The crusta petrosa, in the section which the author was permitted 

 to make, was more than three-eighths of an inch thick. The corpus- 

 cules were very numerous, and generally within the distance of one 

 diameter. The granulated compartments of which the corpuscules 

 were composed, were very visible, and often without radiating fibres, 

 but where these occurred they were of a coarse structure. The 

 transverse independent fibres were beaded in coarse, somewhat tor- 

 tuous, ovoid compartments, and ranged very close to one another, 

 with interfibral spaces of about only two fibral diameters, and followed 

 a straight, perpendicular and parallel course to the surface. The 

 cells of the semi-transparent interfibral space were generally visible. 



The appearances presented by the ivory at its junction with the 

 crusta petrosa, Mr. Nasmyth was unable to ascertain j but in the 

 substance of the ivory the fibres undulated, and their beaded com- 

 partments had a rounded shape : these fibres were frequently in- 

 vested with an irregular congeries of granules distinct from the inter- 

 fibral cells. Towards the central portion of the ivory the compart- 

 ments forming the fibre were frequently so disposed as to give the 

 fibre a peculiar tortuous appearance. 



The peculiarities of the tusk of the Missourium are given by Mr. 

 Nasmyth as follows ; and, he says, they would certainly indicate a di- 

 stinct species of Mastodontoid animal: — 



1. The great extent of the crusta petrosa. 2. The close aggre- 



