BRACHIOPODA. 
321 
that the embryonic shell or protoconch (protegulum) of the bradiiopod 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 Swan ton, Vermont,| 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 Lingulops andLiNGUCASMA toTRiMERELLA, 
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 umisually complete 
and conclusive. Various intermediate stages have also been indicated by which 
a similar resultant is attained from the primitive obolelloids through Lakhmina, 
* Beecher ; Duvelopnii'nt of Ibe Brachiopoda, Part I, Introduction ; American Joui'iial of Science, vol. 
xli, p. 344. 1891. 
t In a later work Mr. Walcott ha'i concluded that the Sw.anlon fossil is sufficiently distinct from the 
typical Oholus (oi' Kutorgiiia) Lahradoricus to reqiiii’e a new designation, and hastheridore termed it Kid.or- 
gina Lahradorica, var. Swantonensis (See “Fauna of the Lower Cambiaan Tenth Ann. Kept. Directoi- 
U. S. Geological Survey, pi. Ixiv, figs 2, 3, dated 1890, issued 1892). The figures given in the work cited show 
that the var. Swantonensis is in many respects the more primitive type, its valves being the more nearly 
equiconvex, its surface chai’acters simple concentric strise, while in ihe typical 0. Lahradoricus, W\&vq \s a 
conspicuous elevation of the umbo of the pedicle-valve, a low median sinus on the brachial valve, as well as 
indications of I’adial plications about the beak ; all these are secondary characters which indicate progress 
toward the true Kutorgiita {K cingulata). It secmi evident 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 Swantonensis, 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 Lingula and 
Trimerella, and in the following pages. 
