DAVIDSON — PAL^ONTOLOGICAL NOTES ON THE BRACHIOPODA. 105 



The external surface varies according to the species. In some it is 

 almost smooth ; in others it was longitudinally and finely striated, or 

 coarsely costated, as well as intersected by numerous concentric 

 wrinkles, or lines of growth. All the specimens appear to have been 

 furnished, more or less, with tubular spines. In some Producta, 

 Sfrophalosia, and Aulosteges, both valves were so ornamented ; while 

 in others they were restricted to the ventral valve. In certain species 

 they are smallj delicate, and so closely packed as to conceal every por- 

 tion of the shell, with the exception of the area ; while in others they 

 were irregularly scattered, and chiefly confined to the auriculate 

 portions of the valves. In certain species the spines exceeded by four 

 or five times the length of the shell ; and while some were almost as 

 delicate as the hair of one's head, others exceeded a line in diameter ; 

 the dimensions of the shell, however, had nothing to do with that of 

 the spines j for in some small species these were few and large, while 

 the reverse has occasionally been found to be the case with species of 

 the largest dimensions. Chonetes alone appears to have differed from 

 Produda, Aulosteges, and Strophalosia, in its tubular spines, which are 

 in all known species confined to the cardinal edge of the ventral 

 valve, where they are regularly disposed and interspersed, generally 

 increasing in length as they approach the extremities of the shell. 

 The intimate structure of the shell has been described by Dr. Car- 

 penter, in the second chapter of the " General Introduction " to my 

 work on British Fossil Brachiopoda, and from which I will extract the 

 following passage : — In all the genera of this family large perfora- 

 tions exist, resembling those of Strophoynena depressa in their general 

 aspect, and in the infundibular arrangement of the laminae of the shell 

 around them. Where the shell is furnished with spines, as is especially 

 the case with Producta horrida, the perforations are continued 

 into them ; and such passages are of more than the average 

 dimensions." 



These are the more important external features presented by the 

 family. We will now examine the interior disp)ositions, and will com- 

 mence with those which relate to the smaller or dorsal valve. 



In the Productidce the internal surface of this valve is more or less 

 convex, and presents in the middle of the hinge-line a prominent 

 bi-lobed or tri-lobed projection, which has been termed a "cardinal 



