BRAcHIOPOD:. 909 
In the chapter upon the “Development of the Shell” it has 
been shown that the deltidiwm and deltidial plates, though 
similar in function, are profoundly distinct both in origin and 
structure. Tae former is primitive and fundamental, the latter 
wholly secondary; a replacement of, but never a derivative from 
the former. They may resemble each other, as the so-called 
pseudodeltidium of Cyrrrna is externally like the true deltidium 
of Cxrramsonitrs, StropHomeNA and Rarinesquina, but this is 
purely a superficial similarity. The pseudodeltidium consists, in 
its early condition, of discrete deltidial plates and in its mature 
state it expresses simply a firm coalescence of those plates. The 
term pseudodeltidium is a convenient one to employ for this 
peculiar phase of the deltidial plates; but we have proposed to 
distinguish the latter generally by the use of the term deltariwm 
in application to the parts as a whole, whether coalesced 
(Cyrtina, Nuoteosprra, Rerzta, etc.), or discrete, and deltaria in 
referring to the component plates. The reader is again referred 
to the chapter already cited for the full explanation of the 
differences in the deltidium and deltarium as worked out 
by Bescuer, in which it is demonstrated that the former is, 
in a certain sense, a third valve, not forming on the 
mantle lobes of the young as do the true valves, but upon 
the body of the embryo, enclosing the cardinal space between 
the latter. This solid, continuous, never divided plate or pedicle- 
sheath remains throughout all growth-stages in a large division 
of the Articulate genera mostly of early age, while in a coexten- 
sive group predominating throughout the later history of the 
class, this primitive structure is resorbed at an early stage of 
growth, and the enclosure of the pedicle effected by the forma- 
tion of discrete secondary plates which originate from the mantle 
lobes and not from the body of the shell. 
The term spondyliwm has been applied to the spoon-shaped 
plate which frequently occurs in the pedicle-valve of many of the 
Articulates (Prenramerus, Cxurramponitss). A plate of similar 
character appearing in the brachial valve of the same groups of 
genera has been referred to under the same name, but it is found 
that these plates in the different valves are similar neither in 
origin or function; hence that of the brachial valve is now distin- 
guished by the term cruralium. In the Inarticulate Brachio- 
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