BRACHIOPODA. 307 



of the shell, and in the cardinal area and articulating apophyses, and may 

 be regarded as of some significance in determining the source and point of 

 departure of the productoid stock. CEhlert has demonstrated the existence 

 of reniform or hook-shaped brachial ridges in the genus Stropheodonta {S. Le- 

 hlanci) ;* and although a feature of extremely rare occurrence in that group, 

 it should be given full value in its bearing upon the same question. It has 

 been remarked by many observers that in the earlier forms of Chonetes the 

 reniform ridges are but faintly, if at all, manifest ; the more distinctly pro- 

 ductoid characters of the genus becoming pronounced only with the advent 

 of Productus. f The peculiar cardinal tubes, which are continued into 

 spines, sometimes of a length equal to that of the valves, constitute a feature 

 found only in this group of shells, including Chonostrophia and Chonopectds, 

 here described for the first time, Chonetina, Krotow, and Chonetella, 

 Waagen. The existence of these passages across the ventral area, opening 

 into the interior of the shell, was first observed by Keyserling,J and was dis- 

 cussed at length in Volume IV of the Palaeontology of New York,§ and more 

 recently by Mr. John Young, in Davidson's Appendix to his Supplement on 

 the Brachiopoda.|| Fine hair-like spines are sparingly scattered over the striae 

 in C. papilionacea, Phillips, of the Carboniferous limestone, and traces of similar 

 processes have been mentioned by various writers for other species.TI 



It is probably true in many cases that the supposed bases of the spines are 

 but the coarse punctations of the inner laminge of the shell-substance, exposed 

 by the abrasion of the impunctate outer or epidermal layer. Furthermore, the 

 Chonetes papilionacea is a species whose generic affinities are still debatable. 

 The shell is of immense size, far exceeding that of any other form referred 



* Annales des Sciences Geologiques, vol. xix, p. 63. 



t ffiHLBRT observes "que les inodiiications lUi genre Chonetes ont lieu tout particuliereraent a I'epoque 

 carbonifere, et que leur tendance dans certaines especes, a de riipprocher de la forme producto'ide serable 

 coincider avec I'appai-ition et la d6veloppement maximum du genre Proddctds. Les Chonetes d6voniens, et 

 surtout ceux qui appartiennent a I'epoque silurienne, presentent des caracteres plus nettement d6finis et plus 

 facilement reconnaissables." 



X Geogn. Beobachtungen auf einer Reise in das Petscliora-land, p. 213. 1846. 



§ " " " " p. 117. 



|] " " " " p. 281. " 



*i See Davidson, Carboniferous Brachiopoda, p. 188, pi. viii, fig. 8 b. 1872 ; Waagen, Salt-Range Fos- 

 sils, genus Chonetes. 



