BRACmOPODA. 



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. 263.. 



Fl<i. 260. 



Fig. 267. 



Fig. 268. 



Fio. 270. 



Fig. 2n. 



Fig. 

 Fig, 

 Fig. 

 Figi 

 Fis. 



Fig. 



Fig. 269. 



265. Deltlij-riam of a young Ithynchondla. 



266. Tlie same, at a later stage, willi two triangular delt.aria. 

 2G7. Tlie same, at completed growtli of the deltaria. 



. 263, 20U. Dorsal and prolile views of Magellania Aavescens ; sliowing deltariaand |>cdicle. 



270. Dorsal view of the unibonal portion of an adult TerebratuUna septcntriotuills, with the shell removed by 



acid; showing slight secondary extensions of tho ventral mantle at the base of the pedicle, small deltaria 

 only being formed in this species. 



271. A similar jireijaratiim ot Mnijellania fiatescens ; showing tho complete envelopment of the base of tho 



pedicle by secondary expansions of the ventral mantle, which have formed the deltaria, as shown in 

 fig- 208. (Beeciieb.) 



These plates may unite along the median line, obliterate the foramen, or even 

 extinguish all trace of their original division, as irequently 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 inti-oduction of the term that these processes in the two valves, though 



* Beecher, loc. cit. 



