Spire-Bearing Brachiopoda. — Schu'chert. 103 
of the primitive loop of the two other families. These types 
may be defined as follows : 
1. Primitive loop a simple band, variously situated, be- 
coming in the higher forms more or less V-shaped. Present 
in Zygospirina}, Dayinai, Suestfiince, and Uncitinai. 
2. Loop a simple band in young or immature stages, but at 
maturity more or less medially absorbed, and eventually only 
represented by two short prongs, one attached to each primary 
lamella. Characteristic of Atrypina. and Trigonotretince. 
3. Loop V-shaped, with its apex more or less drawn out 
into a simple or terminally modified extension, which is but 
rarely bifurcated. Present in Anoplothecina>, Rhynchospirina 
and Hindellinoe. 
4. The loop process always present with the distal end bi- 
furcated. The branches are of variable lengths and either 
terminate freely between the first and second volutions of tin- 
spirals, or may continue with these to their outer ends. 
Present in Athyrinw, Diplospirince, and Koninckinince. 
5. The branches of the process remain between the spirals, 
recurve and join near their origin, thus resembling the han- 
dles of a pair of scissors. Present in Meristellinm. 
The ontogen3 r of Zygospira indicates that the Helicopegmata 
was the last to appear and had its origin in the Ancylobra 
chia or terebratuioids. The suborder first manifests itself as 
far as known in the lower portion of the Lower Silurian. It 
attains its climax of differentiation in the Devonian, with 
another outburst of specialization in the Triassic chiefly 
among the double-spired athyroids, while there is rapid de- 
cline and final extinction in the Jurassic. Of the eight sub- 
orders into which the class Brachiopoda is now divided, only 
one, the Trullacea, or pentameroids, became extind before the 
Helicopegmata. These two suborders are the only extinct 
ones of the class. The other six groups still have living rep- 
resentatives, are widely distributed, and are represented by 
about one hundred and thirty species. 
Suborder HELICOPEGMATA. Waagen L883. 
Spiriferacea, Waagen 1883. 
Brachiopoda in which the two spirally enrolled brachia have 
internal calcified supports which are more or less completely 
