142 C. E. Beech er — Development of the Brachiopoda. 



From the facts, that young individuals of paleozoic species 

 belonging to such genera as Zygospira, Spirifer, Orthis, Rhyn- 

 chonella, and Scenidium, have been observed by the writer to 

 retain their original relations to the objects of support, and 

 that casts of the pedicles of fossil Lingulse and Eichwaldia 

 have been described (Davidson, 6 Walcott' 23 ), it cannot be as- 

 sumed that the free swimming condition was ever present in 

 nealogic or ephebolic individuals. Evidently it has always 

 been a larval character. 



Origin of the deltidhim and deltidial plates. 



The origin and significance of the deltidium* (="pseudo- 

 deltidium ") are made apparent in the development of Theci- 

 dium, and it may be well in this place to make a few obser- 

 vations on the genesis of this important character, and its 

 relations to the deltidial plates of other genera, as Phyncho- 

 nella and Terebratula. It has been already noted (Part I), 

 that the deltidium in all species possessing it (the Protremata) 

 is an embryological, or nepionic feature, which may or may 

 not continue to the ephebolic period ; while the deltidial plates 

 in other brachiopods (the Telotremata) appear later during the 

 nealogic and ephebolic periods, or may never be developed. 

 The detailed researches of Kovalevski on Cistella and Thecid- 

 ium, together with other observations now first made, furnish 

 data for a clear understanding of these differences.f 



Figure 18 represents a dorso-ventral section of a ripe cephal- 

 ula just before the transformation, and shows the unequal lobes 

 of the mantle, v being the ventral lobe, and d the dorsal ; h is 

 the head, and p the caudal segment developing into a pedicle. 

 A deposit of integument representing the shell has formed on 

 the inner side of the dorsal mantle lobe [ds), and also on the 

 adjacent dorsal side of the body lobe (del). A larva somewhat 

 more advanced is represented in figure 17, as viewed from the 

 dorsal side. The mantle lobe is still directed posteriorly, as in 

 the preceding figure, and the underlying shell plate is shown 



* The single plate or covering to the triangular opening beneath the ventral 

 beak should be termed the deltidium, as it was thus extensively used by Davidson. 

 When it consists of two plates they may be called deltidial plates. These names 

 have been loosely used. In Part I of this paper the deltidium proper is referred 

 to as pedicle covering, pedicle sheath, and pseudo-deltidium. Hall and Clarke 

 have proposed to call the triangular opening in the beaks of brachiopods, the 

 detlhyrium, and the concave plate in the ventral beak of Pentamerus, Orthisina, 

 etc , they have termed the spondylium. There yet remains a term for the convex 

 plate covering the opening below the beak of the dorsal valve, and resembling 

 the deltidium of the opposite valve. For this feature the name chilidium (je^of) 

 is here proposed. 



f Kovalevski. 15 For Thecidium consult the explanation of PI. IV, figs. 15-26. 

 For Cistella, PI. I, figs. 13-15 ; PL II, figs. 11, 19-21. 



