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 rliynchonellids 

 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 Camakot(echia 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, Avithout 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 decline 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, Orthis loricula 

 and 0. dejleda of the Trenton group, and 0. ? laurcniina of the Hudson River 

 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, Streptorhynchus, etc., into Lept^na, 

 Rafinesqoina, Stropheouonta, Plectambonites, Choxetes and Productus. 



