BRACHIOPODA. 
335 
of two lamelliB, 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. The 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 Eichwaldia 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 Aulocorhynchus 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 Dicam.yra 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 foi-med 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 the Brachiopoda, have a highly diverse origin in different cases. In most 
instances, except where bearing spondylia, they are evidently of muscular origin and surfaces of muscular 
attachment, as shown in SpiR[PERrtfA (see figure 42, page 53, and remarks in foot-note, Part I, p. 49) ; while 
in the Tbimerellidje they appear to be the residuum left by the resorption of a thick testaceous deposition 
about and beneath the area of muscular insertion. 
