PTEROPODA. 



809 



the apex is often partially filled up with a secondary calcareous 

 deposit, or may be partitioned off by one or more curved transverse 

 plates (fig. 725, a). The initial portion of the shell may be smooth 

 and without annulations, and may be simply pointed. In other 

 cases, the apex is distinctly bulbiform (fig. 719, c), 

 its starting-point being a dilated chamber, which 

 appears in no respect to differ from the " em- 

 bryonic shell " of Clio and other Pteropods. The 

 whole of the tube, except the initial portion, is 

 annulated by abruptly elevated rings, which be- 

 come more remote as the aperture is approached ; 

 and the surface between the annulations is marked 

 by fine encircling striae, sometimes accompanied 

 by longitudinal strise as well. 



As regards its minute structure, thin sections 

 show that the tube of Tentaculites is composed 

 of two distinct layers of a different nature (fig. 

 725, b and c). The outer layer is very dense, and is thickened 

 at intervals so as to form the annulations which are so character- 

 istic of the genus. The inner layer is composed of transparent 

 calcite, traversed by parallel lamellae of a dark colour, which are 

 approximately parallel to the surface of the tube. Both layers 



Fig. 724. — Tenta- 

 culites ornatus. Si- 

 lurian. 





Fig. 725. — Minute structure of Tentaculites attenuatits, from the Devonian rocks (Hamilton 

 Group) of Canada, a, Section of the apex of a tube, showing curved transverse partitions ; 

 b, Transverse section of the tube (the oval form of the tube is the result of pressure) ; c, Ver- 

 tical section, taken through the middle of the tube ; d, Vertical section traversing the wall of the 

 tube, and showing its peculiar tubulated structure. The figures are enlarged about twenty times. 

 (Original.) 



are more or less extensively penetrated by a system of exceedingly 

 minute tubuli (fig. 725, d), the direction of which is approximately 

 at right angles to the surface. 



The genus Tentaculites ranges from the Ordovician to the De- 

 vonian, occurring for the most part in limestones, and being often 

 present in vast numbers in particular beds. In no case is the shell 

 attached to any foreign body. 



The affinities of Tentaculites cannot, in the present state of our 

 knowledge, be determined with absolute certainty. The compara- 



