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 Kittorgina 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 cliasm 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- 



* Bebchkr, Amei-ican Journal of Science, voL xliv, p. 138. 1S92. 

 t Bbbchkb, loc. <nt. 



