BRACHIOPODA. 
337 
Under the guidance of the structural features above considered, the main 
lines of derivation of the Articulate genera are more readily apprehended. 
The earliest known representatives of a given group of genera are not 
always the most primitive in structure. In the instance cited in the pre¬ 
ceding paragraph, Orthorhynchula Linneyi is perhaps, by itself considered, the 
closest expression of the fundamental stock from which the rhynchonellids 
have been derived, but it is by no means the earliest of the group. It is known 
only in the latest fauna of the Lower Silurian, while in the earlier faunas, 
Protorhyn CHA, Riiynchotrema and Camarotcechia have attained an abundant 
development. Orthorhynchula either represents a resumption of the primitive 
type, subsequent to such modifications as appear in the earlier rhynchonelloid 
genera, or a continuance of that type, without modification, through preexist¬ 
ing forms as yet unknown. Such instances could be multiplied, as facts of 
similar import are constantly recurring, and a careful consideration of the stage 
of development or declino of each separate and individual organ is requisite to 
determine how far the organism in question is a direct or modified outcome of 
the fundamental type; or a degenerate or senile relapse, after modification, to 
phyletic immaturity. 
The most elementary structure, then, observable among the Articulate 
Brachiopods, is the combination of the deltidium with a distinct pedicle-cavity, 
whose anterior margins are not free, and whose lateral walls or dental lamellae 
are not highly developed; these features being accompanied by gently and un¬ 
equally biconvex valves, well defined cardinal areas and elongate hinge-line; 
producing, in effect, a generally orthoid expression both of interior and exte¬ 
rior. This is the condition of Billingsella of the Cambrian, Orihis loricula 
and 0. dejiecta of the Trenton group, and 0. ? laurentina of the Hudson Biver 
fauna, and it is continued without essential modification, except in the gradual 
contraction of the pedicle-cavity and deltidium, into Strophomena of the Silu¬ 
rian, its allies and successors, Orthothetes of the Devonian, and Derbya of the 
Carboniferous, Hipparionyx, Triplegia, Streptorhynciius, etc., into Lept^na, 
Rafinesquina, Stropheodonta, Plectambonites, Ciionetes and Productus. 
