282 APPENDIX TO THE SUPPLEMENTS TO 



mineral matter, which, being generally darker in colour than the substance of the shell, 

 reveals the path of the tubes through the shell in the clearest manner. It is well known 

 that the scries of cardinal spines in Chonetes diverge from the beak on either side of the 

 valve, in a slanting manner towards the outer extremity of the shell, but I do not think 

 it is known that the tubes in their inner path through the shell slanted in the opposite 

 direction, so as to point or converge towards the beak [PI. XX, fig. 20]. This is beau- 

 tifully shown in some of the etched specimens I have prepared. In these it is seen that 

 the spines nearest the beak slant inwards at the greatest angle, the inward convergence 

 of the tubes decreasing on either side towards the outer edge of the shell. When the 

 tubes, however, reached the cardinal edge of the shell and were prolonged outwards in the 

 form of spines, instead of continuing to converge towards the beak, they became suddenly 

 bent in the opposite direction, so that they afterwards diverged from the beak in a slanting 

 manner." [The existence of tubes traversing the thickness of the area was noticed for 

 the first time by Count Keyserling. 1 ] 



" In bringing this note to a close, I have only to remark that the points of interest 

 noted in my investigation of the shell-structure of Chonetes Laguessiana are four ; these 

 are — 



" 1. The occurrence on the ribs of both valves of a series of wide-set tubular openings, 

 or bases of spines, that do not pass inwards through the thickness of the shell [fig. 21 e\. 



" 2. A row of very minute, close-set pores that are placed along the central line of 

 each rib, but which descend only a very short distance into the shell-substance and then 

 disappear [fig. 21/]. 



" 3. A series of wide-set perforations, not visible at the surface, but which start near 

 the middle layer of the shell and are continued in single rows between each rib in an 

 upward, slanting direction through the shell, and appear upon the inner surface of the 

 valves as numerous raised tubercles [fig. 20 d\. 



" 4. A row of tubular spines in the cardinal edge of the ventral valve, that open with 

 round orifices on the inner surface of the shell, the tubes at first converging towards the 

 beak, but on arriving at the outer edge being prolonged as spines and bending suddenly 

 in the opposite direction." 



I also sent Mr. Young for examination specimens of Mr. (Ehlert's three species of 

 Chonetes ; C. plebeia, C. tenicostata, and C. sarcinulata, and although he found their 

 shell-structure much altered and obscured, still he could discern in them numerous 

 depressed and slanting tubercles and spines (visible on the inner surface of the valves 

 and which, as in C. Laguessiana, did not traverse its entire thickness). He repeated his 

 observations on specimens of Chonetes armata from the Devonian Rocks of Ferques. 

 No visible perforations were to be seen on the outer surface of the valves, but after 

 etching the valves with weak acid, he found that the perforations did not begin to show 



1 ' Geogn. Beobacht. auf Einer Reise in das Petschora-Land.,' p. 213, 1846. See also (Ehlert, ' Bull. 

 Soc. Geol. France,' 3rd ser., vol. xi, p. 526, 1883. 



