Mr. John Young — -On the Hinge of Spirifer. 19 



It had long been noticed that in the shell structure of the external 

 surface of the hinge-line area in the ventral valve of many of the 

 species of Spirifera, as well as in others of the straight-hinged 

 Brachiopods, there exist a series of vertical lines or tubes, slightly 

 inclined and waved in some specimens, but more often straight 

 in others. These lines are well illustrated in the figures of many 

 of the Spirifera in the plates of Dr. Davidson's Monograph of the 

 Carboniferous Brachiopoda, and in the Spirifera figured on plate ix. 

 of Prof. King's Permian Monograph, but so far as I am aware, no 

 one has noticed or described these lines or tubes as forming and 

 corresponding with a row of denticles along the edge of the valve in 

 the specimens on which they are found. I Lave, however, obtained 

 clear evidence in the specimens that I have examined that such, is the 

 case, as shown in the diagram-sketcb of the hinge-area. Some of 

 the lines bifurcate in their upper extremity, but the greater majority 

 are single, and they slightly increase in thickness as they approach 

 the edge of the hinge, in which they are seen to be continuous with 

 the denticles. 



At first, I thought that these lines leading to the denticles might 

 have originally been hollow tubes, but I have failed in finding any 

 of them filled with clay, pyrites, or other mineral matter, which is 

 common in the tubes and perforations of other shells from the same 

 beds. Such being the case, I am now inclined to regard them as 

 having been originally composed of vertical lines of aragonite in 

 the shell structure, which, being harder than the ordinary calcite of 

 the shell, went to the formation of the row of denticles. These lines 

 are now seen to be composed of a coarser and more structureless kind 

 of calcite, than that forming the hinge-area of the shell, and it often 

 weathers out into hollow grooves upon the external surface, or when 

 the lines have been etched with acid. 



It is now well known that both calcite and aragonite enter into 

 the composition of the shells of many of the Mollusca, and it has 

 been further shown, by Dr. Sorby and others, that while the ara- 

 gonite was originally the hardest of the two varieties of lime present, 

 yet, during fossilization, it was the first to change or go to decay, 

 its place being often taken by calcite that had infiltrated into the 

 shell, and when this is the case, it is found that the calcite replacing 

 the aragonite is of a more crystalline and coarse structure than that 

 now entering into the composition of other parts of the shell. If I 

 am therefore right in my conjecture as to the denticles and lines 

 of coarser structure seen on the hinge-area of the ventral valve of 

 the Spirifera under notice having been aragonite, its present con- 

 dition can be explained by the changes that have taken place during 

 fossilization, and of its former use in the structure of the shell, 

 while the animal was in life, in the formation of its hard row of 

 small denticles. 



After a careful examination of the hinge-area of the dorsal valve 

 in this species of Spirifera, I am inclined to think that the row of 

 pits or sockets seen along its area have been worn out solely by 

 the friction of the harder projecting points of aragonite in the 



