DEVELOPMENT OF THE BllACUIOPODA 257 



writer to retain their original relations to the objects of sup- 

 port, and that casts of the pedicles of fossil Lingulye and 

 J^'ickwalJia have been described (Davidson,^ Walcott^^), it 

 cannot be assumed that the free-swimming condition was 

 ever present in neanic or ephebic individuals. Evidently it 

 has always been a larval character. 



Origin of the Deltidium and Deltidial Plates. 



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

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

 cidium, and it may be well in this place to make a few 

 observations on the genesis of this important character, and 

 its relations to the deltidial plates of other genera, as 

 Rhynchonella and Terehratula. It has been already noted 

 (Part 1), that the deltidium in all species possessing it (the 

 Protremata) is an embryological, or nepionic feature, which 

 may or may not continue to the ephebic period; while the 

 deltidial plates in other brachiopods (the Telotremata) appear 

 later during the neanic and ephebic periods, or may never 

 be developed. The detailed researches of Kovalevski on 

 Cistella and Thecidium, together with other observations now 

 first made, furnish data for a clear understanding of these 

 differences. J 



Figure 102 represents a dorso-ventral section of a ripe 

 cephalula just before the transformation, and shows the un- 



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

 beak should lie termed the deltidium, as it was thus extensively used by David- 

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

 names have been loosely used. In Part 1 of this paper the deltidium proper is 

 referred to as pedicle covering, pedicle-sheath, and p.seudo-deltidium. Hall and 

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

 the delthi/rium, and the concave plate in the ventral beak of Pentamerus, 

 Orlhisina, 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 (xfi'^os) is here proposed. 



t Kovalevski ^^ For Thecidium consult the explanation of pi. iv, figs. 15-26 ; 

 for Cistella, pi. i, figs. 13-15 ; pi. ii, figs. 17, 19-21. 



17 



