BRACHIOPODA. 335 



of two lamelloB, each representing one of the coalesced or adherent crural 

 plates.* 



The unsupported convex internal plate or " shoe-lifter " in the pedicle-valve 

 of Merista and Dicamara must be interpreted as an entirely different structure 

 from the spondylium. It is not produced by convergent dental plates, but 

 these, on the contrary, are divergent, the arched plate uniting its inner edges. 

 Its origin and the reason of its existence are still obscure. Tlie readiness with 

 which the filling of the cavity between this plate and the outer wall of the 

 valve separates from the shell, carrying with it the enclosing walls, leads to the 

 suggestion that the " shoe-lifter " may be the innermost lamina of the shell 

 separated from the rest of the valve and leaving it thinner in this region. This 

 plate, upon its convex surface, bears the muscular bands, in whole or in part. 

 In EiciiwALDiA it has been observed that the small internal plate of the pedicle- 

 valve is probably a modified condition of the deltidium, as the pedicle passes 

 beneath it, while the platform in AcLOcoRHYNCiins may prove to be wholly of 

 muscular origin. 



The compound " shoe-lifter," divided by the median septum in the brachial 

 valve of Dicamara, is like the corresponding plate n the pedicle-valve in hav- 

 ing no connection with, or origin from the articulating apparatus. This plate 

 is not a cruralium, and in precisely the same sense that the simple " shoe-lifter " 

 is not a spondylium. Such cases as Merista and Dicamara are, therefore, not 

 to be cited as examples of the concurrence of spondylium and cruralium, with 

 the secondary condition of the pedicle-covering or deltarium, but are, rather, 

 illustrations of the production of parts which may be similar in function in the 

 mature condition, but are totally distinct in origin; in other words, interesting 

 instances of morphic equivalents. 



* In the pentameroids the median septum of the pedicle-valve supporting the spondylium, is formed in 

 a similar manner by a continuation and coalescence of the dental plates, and wherever the median support- 

 ing septum exists in this group, it will probably be found to have this composition. Median and lateral 

 septa, however, in the valves of ihe Brachiopoda, have a highly diverse origin in ilifferent cases. In most 

 instances, except where bearing spondylia, they are evidently of muscular origin and surfaces of muscular 

 attachment, as shown in Spirikerijia (see figure 42, page 53, and remarks in foot-note. Part I, p. 49) ; while 

 in the Trlverellw^ they appear to be the residuum left by the resorption of a thick testaceous deposition 

 about and beneath the area of muscular inseition. 



