BRACHIOPODA. 321 



that the embryonic shell or protoconch (protegulum) of the brachiopod is " semi- 

 circular or semi-elliptical in outline, with a straight or arcuate hinge-line, and 

 no hinge-area. A slight posterior gaping is produced by the pedicle-valve 

 being usually more convex than the brachial."* It appears, furthermore, to be 

 composed of corneous, impunctate shell-tissue. The same investigator finds that 

 the species described by Billings as Obolus Labradoricus, from a horizon at 

 L'Anse au Loup, now regarded as Lower Cambrian, and subsequently identified 

 by Walcott, at the same horizon at Swanton, Vermont,f is the nearest approach 

 of the adult brachiopod to the simple type of the protoconch ; a semicircular 

 corneous shell, with gaping cardinal margins. This shell has been distinguished 

 by the generic term Paterina. 



There are, undoubtedly, other brachiopodous shells of obolelloid type that are 

 quite as ancient as Paterina ; still the latter exemplifies the line along which 

 the development of more complicated forms has proceeded, and it is in all 

 respects the simplest known brachiopod. Paterina is an embodiment of 

 the predicted ancestor of the linguloids and obolelloids, and, with our present 

 knowledge, it appears to be the radicle of all the brachiopoda, both inarticulate 

 and articulate. 



The departure from Lingula, through Lingdlops and Lingulasma toTaiMERELLA, 

 by the progressive development of the vaulted muscular platform (see Part I, 

 pp. 46, 165, plates i, ii, iva) is confirmed by evidence which is unusually complete 

 and conclusive. Various intermediate stages have also been indicated by which 

 a similar resultant is attained from the primitive obolelloids through Lakhmina, 



 Bbbchkr ; Develo|Min;nt cif the Bracbiopu<l&, Part I, Iiitrcxluction ; American Journal of Science, vol. 

 ill, p. 344. 1891. 



t In a later work Mr. Walcott ha« concluded that the Swanton fossil is sufficiently distinct from the 

 typical Obolus (or Ktitorgina) Labradi>ricu.i to require a new {lesig'ii.ilion, and has therefore termed it H'frfor- 

 ghta Lal>radorica, var. Swantonent'u (See "Fauna of the Lower Camlman;" Tenth Ann. Rept. Director 

 U. S. Geolosrical Survey, pi. Ixiv, fiffs 2, 3, dated 1890, issued 1892). Tht; fif^ures given in the work cited show 

 that the var. SioanfonensU is in many respects the more primitive type, its valves being- the more nearly 

 equiconvex, its sai-face characters simple concentric strije, while in the typical 0. Labraiinricwf, there is a 

 conspicuous elevation of the umbo of the i>edicle-valve. a low median sinus on the l)rachial valve, as well as 

 indications of rarlial plications about the beak ; all these are secondary characters which indicate progress 

 towanl the true KuTOBr.iwA {K cingiilata). It sei-mi eviilent that the generic term Paterina was based 

 upon the Swanton fossil, and hence, if the author'.s intentions are correctly interpreted, the type of the 

 genus is Paterina SwantonenxU, Walcott As to the value to I)e ascribed to differences of shell-composi- 

 tion within a given association of closely related genera, see reniai'ks under the discussion of Linqula and 

 Tbimxrbi.la, and in the following pages. 



