182 Frederick Chapnutn : 



material, the latter being vertically tubulated, and the horny 

 layers devoid of any structure. 



In the Silurian shells here described the sections are of a 

 pale brown colour. A transverse section shows linely laminated 

 structure on the broken edges of the slice. Under a high power 

 the corneous layer is seen to be crowded with minute vermiform 

 canaliculi. They are rather smaller, and more irregular than 

 those in Carpenter's figure (fig. 22, loc. supra cit.), and agree 

 with his example not only in their being set obliquely to the 

 plane of lamination, but also in tending to branch, so that 

 in the same focal plane both the orifices and lengths of the 

 canaliculi are seen (fig. 4). 



A vertical section of the fossil shell was made which, although 

 somewhat thick, owing to its brittleness, shows an alternate, 

 laminate structure quite clearly. The shell is of the same 

 relative weight as that of a living example, and agrees in the 

 number of laminae, which vary in relative thickness according 

 to the part of the shell from which the section was taken. The 

 stoutest layers in the silurian shell measure about .05 mm. in 

 thickness. 



The darker layers in this silurian fossil appear to represent 

 the calcareous laminae, and the lighter layers the corneous. The 

 altered calcareous laminae, when closely examined, show a few 

 distinct traces of vertical tubes, especially towards the limiting 

 surfaces of the layer. The corneous laminae are distingtiished 

 by an obliquely striated or fibrillate structure, very finely cana- 

 liculated, as in a living Lingula shell {L. albida, Hinds), which 

 I have examined in specimens kindly given by Mr, C. J. Gabriel ; 

 although these layers are stated by Gratiolet to be structureless 

 in the living L. anatina. Some of the horny layers of the 

 fossil shell have been invaded by a parasitic boring fungus, the 

 thallus of which shows a tendency to spread in the plane 

 between two sets of laminae, but also penetrates to some depth 

 into the shell. 



This silurian Lingula show^s no trace of carbonate of lime, 

 this mineral having probably been leached out during the long 

 process of fossilisation ; but so gradually has been the replace- 

 ment by other constituents that tHe intermediate horny layers 

 are left undisturbed. In the living JAngula shell the proportion 



