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. 465. . 



Flu. 260. 



Fig. 267. 



Fio. as. 



Fig. 269. 



Fio. 270. 



Fio. 27]. 



ns. SEB. I>elth>'rlain of a yoan; Jihjfnchonella. 



Tig, 266. The same, at a later stage, with two triangular deltaria. 



Fig. 167. Tho same, at coroplctcil growth of tlie tlcltarta. 



Figs. 263, i63. Dorsal and prolllc Tiows of Magellania HaracetUi showing <Ieltarla and pedicle. 



Fig. 370. Doraal view of the ambonal portion of an adalt Terebratulina teptentrionalit, with the shell removed by 



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



only being formed in this species. 

 Fig. 271. A similar |ircparatii>n ol itngellania JUKOceru i 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 



flg. 268. (Beeciieb.) 



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 



* Bbbchbb, 2oc. cU. 



