206 
T. Davidson — What is a Brachiopod ? 
ITegerlia, Kraussina, Terebrirostra, Magas, Äfannia, Boucliardia, 
Platidia, Argiope , Cistella, Zellania, Reusse/aria, Gicynia, Macandrevia 
Diclasma, Megantheris, Slringocephalus, Trepidolopsis and some others. 
Terebratula appeared at tbe close of tke Silurian period, and con- 
tinued to be represeuted up to tbe present time ; but tbe larger 
number of genera liave bad a very limited distribution in time. 
Family 6. Tliecidcedce. — Shells small, tbick, varied in shape, 
attacbed by a larger or smaller portion of tbe sbell substance of tkeir 
ventral valve ; area flat ; deltidium indistinct ; valves articulated ; 
loop in tbe dorsal valve folded into two or more lobes lying in 
kollows of correspondiug shape excavated in tbe substance of tbe 
valve. This loop, or apopkysary ridge, supports tbe brachial inem- 
brane, wbose thickened and cirrated margin is apparently attacbed 
to tbe inner sides of tbe grooves ; sbell structure perforated by 
canals. Only one genus at present known, Thecidium. It appeared 
in tbe Trias, and bas continued to be represented up totbe present time. 
Family 7. Spiriferidce. — Shells variable in shape, ovate, elongated, 
transverse, trilobate, etc., witk tbe kinge-line straight and extended 
into wing-shaped expansions : valves articulated, witb or witbout a 
flattened area in tbe ventral valve : animal free, or attacbed during 
at least a portion of its existence by means of a peduncle, or by 
muscular fibres issuing from an angular or circular foramen in tbe 
beak or area of tbe ventral valve. Dorsal valve intemally furnished 
witb two calcareous spiral processes connected in different manners 
and directed outwards towards tbe side of tbe shell ; tbese processes 
afforded Support to the brachial appendages. This family comprises 
tbe following impunctate and punctate genera : Spirifera, Cyrtia, 
Cyrtina, Spiriferina, Martinia, Atkyris, Meristina, Merista, Bctzia, 
Nucleospira, Trematospira, Rhynchospira, Meristella, Zygospira, Coclo- 
spira, Rlynchotrema, Uncites, Ambocoelia, Charionella , Syringolhyris, 
Eumetria, Suessia, Vetulina (?). Tbe first species belonging to this 
family made its appearance during the Silurian period, and tbe 
family became entirely extinct in tbe Inferior Oolite. 
Family 8. Rhynchonellidce. — Yalves articulated, veiy variable in 
shape, more or less trigonal, often trilobed or ovate, smootb or 
plicated ; foramen beneath a usually produced and pointed beak, 
completed by a deltidium at times concealed : brachial appendages 
flesky and spirally coiled, flexible, and supported only at tbeir origin 
by a pair of short-curved shelly processes, or througkout by two 
broad spirally-coiled lamellte (tbese spires are vertical, closely 
adpressed, and directed upwards towards tbe centre of tbe valve): 
shell structure fibrous and impunctate. This family comprises the 
following genera : Rhynclionella, Atrypa, Eatonia, Leptoceelia, Bra- 
cliymerus, Anastrophia, Leiorhynchus, Camarophoria, Rhyncliopora, 
Rhynchonellina, and one or two others. Tbe first species appeared 
during tbe Silurian period, and representatives of tbe fainüy liave 
continued to tbe present time. 
Family 9. Pentameridee. — Shells ovate, somewhat pentagonal ; 
valves articulated, without kinge-area ; foramen angular ; no delti- 
dium : inside of ventral valve two contiguous vertical septa of 
