560 APPENDIX. 



« sented in Plates XXV. figs. 1, 2, 3. and XXVI. 



" fis*. 7. of the volume above referred to. This tube is 

 " continued through the whole length of the siphon, 

 " and from its present irregular shape appears to have 

 " been composed of a coriaceous substance, capable of 

 " dilatation and contraction. The space within the si- 

 " phon, between its interior walls and the outside of 

 " the included tube, has a number of plates radiating 

 " from the latter, throughout its entire length, and 

 " apparently connecting it with the inner walls of the 

 " siphon ; but these plates are too much covered by 

 " sparry crystallisation to enable us clearly to make out 

 " their character. This tube may have been the organ 

 " into which water could be received, when the animal 

 a required an increase of its specific gravity in order to 

 " descend ; a purpose which is supposed to be served 

 " by the siphon of the nautilus and other chambered 

 " shells. 



" There is also one specimen, which, though not in 

 " good preservation, is doubtless a Catenipora or chain 

 " coral, a genus characteristic of the older transition 

 " limestones, in which beds also, orthocerata are 

 " common." 



General List of Specimens, brought to England by 



Captain Back.* 



From the Athabasca (or Elk) River; (probably from 

 one of the Portages). — Porphyritic, grey, compact felspar, 

 enclosing grains of quartz, and of crystalline felspar. 



* These specimens have heen compared with those in Dr. Richardson's 

 collection, now in the museum of the Geological Society, of which a list 

 is given in the Geological Appendix to Franklin's Second Journey. The 

 numbers of the corresponding specimens in that list are indicated below. 



