BRACHIOPODA. 
327 
or near the extremities of the cardinal line/*^ Such cases indicate, in the text¬ 
ure and composition of the shell, a direct passage from the most primitive 
inarticulate to the articulate type. In this feature only, the connection between 
the two divisions of the class is no closer or more clearly manifested than in the 
instances mentioned, but it has been shownf that Kutorgina cingulata may retain 
a pedicle-covering or external sheath, in fact a true deltidium bearing an apical 
perforation, like that in Clitambonites. A deltidium-like structure is highly 
developed or fully retained at maturity in Iphidea. This is evidence of the 
highest moment, and shows conclusively the line along which the clitambon- 
itoids and strophomenoids have been derived. It is an immediate departure 
from the primitive type of the brachiopod into the articulate subtype. 
Passage from the inarticulate to the articulate plan of structure was thus 
effected at a very early period; indeed, almost at the outset of the history of 
the group. The continuance of the two types has since been that of diverging 
series, constantly widening the structural gap between them. We have no 
evidence that this chasm has been bridged at any other point than near 
its source; the inclinations from the one type toward the other, shown in the 
articulating processes of Bakroisella, Tomasina, etc., represent uncompleted 
accessory lines of development, which were abruptly terminated without 
accomplishing the full transition. Such forms have left no descendants, so far 
as known. 
Before entering upon a summary of the phyletic relations of the genera of 
the Articulata, it is important to apprehend the full significance of the modifi¬ 
cations here appearing in the structure of the pedicle-passage and the surfaces 
upon which the muscular bands are implanted; in other words, the origin and 
development of the deltidium, the deltidial plates, and the spoon-shaped mus¬ 
cular platform, or spondylium, which may occur in either or both valves, and 
may be supported or not supported by a median septum. 
The deltidium and deltidial plates, though similar in function, are profoundly 
distinct, both in origin and structure. The former is primitive and funda- 
* Beecher, American Journal of Science, vol. xliv, p. 138. 1892. 
t Beecher, loc . cit . 
