CHONETES. 117 



In many specimens the spines can be seen to originate near the inner 

 margin of the area, or along the hinge-line, and can be traced in the 

 substance of the shell in an oblique direction inwards, at first nearly 

 parallel with the margins of the foramen ; and it is only after becoming 

 free, that they take a vertical or obliquely outward direction. 



The nature of these tubes penetrating the substance of the shell, has 

 been pointed out by Count Von Keyserling*, and shown in his illustra- 

 tions of Chonetes comoides ; but this function has been doubted by Prof. 

 De Koninck, who regards their obliquely inward direction as a con- 

 trary indication.f In many of our specimens, however, I am able to 

 trace these tubes through the substance of the shell from the hinge- 

 margin (their opening into the interior being a little within the 

 margin), beginning near the triangular foramen, and penetrating the 

 shell to the upper edge of the area in lines parallel to the sides of the 

 foramen. Those nearest the centre are more closely arranged than those 

 at a greater distance : the former appear on the exterior as minute pus- 

 tules, sometimes very close to the apex of the shell ; and in receding 

 from the centre, they become more prominent, and attain the character 

 of spines. In some examples, the obliquity of the tubes within the sub- 

 stance of the shell is seen to become less and less on receding from the 

 apex, and they are often curved outwards before reaching the surface ; 

 but the connexion of these tubes with the spines is unmistakable. 



In numerous examples of the casts left by the destruction of the ven- 

 tral valve, the form of the tubes is well preserved in the infiltrated 

 matter ; and in these casts we often find evidence of the tubes near the 

 centre, where no spines are visible on the exterior surface of the shells 

 of the species, as in Chonetes logani and C. scitula. 



* Geognostiche Beobachtungen auf eiuer Reise in das Pctschora-land : By Count Alexander Von 

 Keyscrling, 1846. 



t " These grooves do not appear to me to be produced by the cardinal tubes, as Mr. Dk Keyserliwo 

 thinks, because in thi.'i case they should take an opposite direction : I am led to believe that they depend 

 solely on the successive growth of the shell." Recherces sur les Animaux Fossiles, p. 190. 



