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 Lahradoricus, 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, undoubteilly, 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 Lingulops and Lingulasma toTRiMERELLA, 

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

 pp. 4G, 165, plates i, ii, \\a) 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 LakHxMINA, 



* Beeoher ; Dbvelniiuh-nl (if llie Bi-achiopoda, Part I, IiitroiUiotion ; Amei-ican .Joiii'nal of Science, vol. 

 xli, p. 344. 1891. 



t In a later woi-k Mr. Walcott ha=i concluded that the Swanlon fossil is sufficiently distinct fi-om the 

 typical Ohnlus (or Kutorglna) Lahradoricus to require a new designation, and has therefore termed it Kiitor- 

 gina Lahradorica. var. Swa7ito)iensk (See "Fauna of the Lower Cambrian;" Tenth Ann. Rept, Director 

 U. S. Geoloijical Survey, pi. Ixiv, figs 2, 3, dated 1S90, issued 1892). The fig-uros given in the work cited show 

 that the var. Sivanfo7ien.iis is in n'.any respects the more primitive type, its valves being the more nearly 

 equiconvex, its surface chai'acters simple concentric strijE, while in ihe typical 0. Labradoriciis, \heye \s a 

 conspicuous elevation of the umbo of the pedicle-valve, a low median .sinus on the brachial valve, as well aa 

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

 tmvard (he ti'ue Kctorgin.*. (S" clngalata). It seemi evident that the generic term Paterix.\ was based 

 upon the Swanton fo.ssil, and hence, if the author'.s intentions are cni-rectly interpreted, the type of the 

 genus is Paterina SwantonensU, Walcott, As to the value to be ascribed to differences of shell-composi- 

 tion within a given association of closely related genera, see remarks under the discussion of Lingcla and 

 Trimerella, and in the following pages. 



