T. Davidson — What is a Brachiopod ? 
205 
Tretexterata, King. 
Family 1. Lingulida. — Shells generally either oblong or circular, 
with a peduncle, sometimes of considerable length, passing out 
between tlie valves or through a narrow channel in the hinge- 
margin ; texture horny and calcareous : no calcified support for the 
labial appendages, the fleshy spiral coils directed upwards. This 
family would comprise the following genera : Lingula. Lingulella, 
Lingulops, Lingxdepis, Glottidia, Monobolina, Obolus, Obolella, Dig- 
nomia, Schmidtia, Acrites, Volborthia, and some others. It may, 
however, be hereafter desirable to group the genera above recorded 
into two groups or families, Lingulidce and Obolidce. Lingulella is 
one of the oldest, if not the oldest type of animal life now known, 
while Lingula appeared for the first time about the middle of the 
Cambrian period, and has continued to be represented up to the 
present time. 
Family 2. Discinida. — Shells more or less circular or oval-shaped, 
attached by a peduncle passing through a foramen in the ventral 
valve ; shell calcareous and horny, setae extremely long, barbed with 
cilia of great length : labial appendages fleshy, curved backwards, 
with small terminal spire directed downwards, as in Crania. Genera : 
Discina, Tremates, Discinisca, Kutorgina (?), Acrotreta (?), Siphono- 
treta (?). Discina appeared about the middle of the Cambrian period, 
and has continued to exist up to the present time. 
Family 3. Craniada. — Shells orbicular or limpet-like, entirely 
free or attached by a greater or lesser extent of the under surface 
of their ventral valve : labial appendages spirally coiled, directed 
towards the bottom of the dorsal valve : shell calcareous, perforated 
by minute canals. Genera : Crania, Craniops, Craniscus, Pholidops. 
The genus Crania appeared for the first time during the Silurian 
period, and has continued to be represented up to the present time. 
Family 4. TrimerelUdce. — Shells transversely or longitudinally 
oval : ventral valve usually the largest and flattest. with a more or 
less developed beak and area : ventral valves generally the most 
convex, hinge rudely or faintly dentated : all the genera are provided 
with a solid or vaulted muscular platform in the interior of both 
valves ; no calcareous support for the labial appendages : shell cal- 
careous, and in two of the genera very massive. All the forms are 
extinct. Genera : Trimerelia, Monomerelia, Dinobolus. The genera 
and species of this family are restricted to the Silurian period. 
Clistexterata, King. 
Family 5. Terebratulidee. — Shells very variable in shape, with a 
prominent beak, truncated by a circular foramen, partly completed 
by a deltidium in one or two pieces ; labial appendages United to 
each other by a membrane, variously folded upon themselves, and in 
some genera spiral at their extremities. These appendages are 
entirely or partially supported by a calcified process assuming a 
great variety of shapes. All the species lived attached to submarine 
bodies by the means of a peduncle. Shell structure perforated by 
canals. Genera: Terebratula, Terebratulina, Waldheimia, Terebratella, 
