BRAoHIOPODA. 911 
or primitive deltidial plate formed upon the body of the embryo 
(prodeltidium). 
The transverse plate or platform in the brachial valve of the 
Inarticulates is evidently homologous in origin and function with 
that of the pedicle-valve in the same group. On the other hand, 
the spoon-shaped process or cruraliwm in the brachial valve of 
the Articulates (PenrameRvs, etc.), is a totally different structure 
from the spondylium, similar in function but of more fugitive 
valve. It has originated from the convergence and union of 
the crural plates, and it may, like the spondylium, rest upon the 
inner surface of the valve or be supported by a median septum. 
The cardinal area is a feature of whi-h we find a trace spas- 
modically among the Inarticulates, and, in the Articulates, is more 
generally developed in the deltidium-bearing genera, though it 
may be said that in this group also it is very irregular in its 
appearance. The genus Spieirer furnishes a striking instance of 
its persistence in the deltarium-bearing shells. It is probable that 
the existence of this area has little fundamental connexion with 
tne condition of the pedicle-coverings. It is a very palpable fact 
that there is a much more intimate relation between this area 
and the general form of the shell; thus in the elongate shells like 
the terebratuloids, meristoids, retzioids and the pentameroids for 
the most part, there is no such area present. Where the form of 
the shell is more generally transverse as among the Orraipz, 
in SrropHomena, CrrramBonirrs, Derpya, Spirirsr, etc., the area 
is highly developed. 
It has been suggested that this area may have originated in an 
obstruction to the peripheral growth of the valves upon their 
posterior margins, and would, therefore, be greatest in shells 
whose pedicle was short and necessitated a close attachment (see 
pp. 155, 163). 
In this presentation of the characters of the generic groups, 
the genus Lineura has been taken as a starting point, more for 
the reason that it is a widely distributed and well-known type 
than because we have any reason to regard it as primitive. 
The nearest approach to the radicle of the entire class is 
expressed by the genus Paterna, Beecher (p. 247), which bears 
at maturity the form and structure of the primitive sheli or pro- 
tegulum. It is orbicular in form, with a straight or arcuate 
163 
