BRACHIOPODA. 
331 
found that these plates are derived from two secondary expansions of the man¬ 
tle of the pedicle-valve enveloping the base of the pedicle.^ These manifest 
themselves only in later or post-larval growth-stages, and as they are a product 
of the mantle lobes, may partake of the same punctate structure as the valves. 
Fig. 265. Delthyrium of a young Jthynchonella. 
Fig. 266. The same, at a later stage, with two triangular deltaria. 
Fig. 267. The same, at completed growth of the deltaria. 
Figs. 263, 269. Dorsal and profile views of Magellania ilavescens; sliowing deltaria and pedicle. 
Fig. 270. Dorsal view of the umbonal portion of an adult Terebratulina septentrionalis, with the shell removed by 
acid; showing slight secondary extensions of the ventral mantle at the base of the pedicle, small deltaria 
only being formed in this species. 
Fig. 271. A similar preparation of Magellania jlavescens; showing the complete envelopment of the base of the 
pedicle by secondary expansions of the ventral mantle, which have formed the deltaria, as shown in 
fig. 268. (Beecher.) 
These plates may unite along the median line, obliterate the foramen, or even 
extinguish all trace of their original division, as frequently seen in Spirifer, 
Cyrtia and Cyrtina (pseudodeltidium), thus simulating in every respect the true 
deltidium; though it is now evident that these and the deltidium are of funda¬ 
mentally different nature. These structures, then, become, at once, a most 
important basis of classification among the articulate Brachiopods. 
In this work the term spondylium has been applied to the spoon-shaped plate 
which, when present, is usually found in the pedicle-valve only, but among the 
pentameroids frequently occurs in both valves. It has become evident since 
the introduction of the term that these processes in the two valves, though 
* Beecher, loc. cit. 
