THE GENUS CHONETES. 



243 



Hamilton and Chemung groups. Several species are known in the Carbo- 

 niferous limestones and in the Coal measures. 



For the most part the species are of small size, and, when well preserved, 

 it is not very difficult to distinguish them ; but in their various conditions 

 of preservation, such as partially exfoliated and more or less worn shells, 

 casts of the interior and impressions of the exterior in a matrix varying from 

 pure limestone to calcareous shale, and through the arenaceous shales to a 

 nearly pure sandstone, the specific relations often become difficult of 

 determination. 



In a very large proportion of the specimens coming before me for investi- 

 gation, the cardinal spines cannot be seen, or are but imperfectly 

 preserved. Their comparative strength or direction often furnishes means 

 for specific distinction ; but I have not been able to satisfy myself that the 

 number of spines on the hinge-line is of specific importance, for these are 

 sometimes unequal on the two sides of the apex, and their development 

 seems often to be promoted or retarded by the irregular growth of the 

 shell ; some of the larger species having only three or four spines on each 

 side of the beak, while in the smallest specimens of the group in our 

 rocks (C leyida), I am able to count six spines on each side ; and in 

 C. scitula, as many as six or seven, the bases only of many of them being 

 preserved, in the substance of the shell, as tubular openings. 



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 accompanying figure of the hinge-line of Ckonetes scitula, greatly 

 enlarged, will illustrate the relations of the tubular spines. 



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

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

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

 De Koninck, who regards their obliquely inward direction as a contrary 

 indication.! In many of our specimens, however, I am able to trace these 

 tubes through the substance of the shell from the hinge-margin (their 



* Q-eognostiche Beobachtungen auf einer Keise in das Petschora-land : By Count 

 Alexander Von Keyserling, 1846. 



f ^' These grooves do not appear to me to be produced by the cardinal tubes, as M. 

 De Keyserling thinks, because in this case they should take an opposite direction : I 

 am led to believe that they depend solely on the successive growth of the shell.'" 

 Recherches sur les Animaux Fossiles, p. 190. 



