ORDER IV TELOTREMATA 337 
Metaplasia, Hall and Clarke. Spirifer-like shells, with a median fold on 
the ventral valve and a sinus on the dorsal valve. Lower Devonian ; North 
America. 
Family 3. Athyridae. Phillips. 
Spiriferacea with the bases of the primary lamellae situated between the spiralia, 
and sharply recurved dorsally at their junction with the erura, Spiralia more or less 
laterally directed. Jugum complete, V-shaped, with the apex drawn out into a simple, 
bifurcated, or otherwise modified process. Silurian to Trias. 
This large, chiefly Palaeozoic family, with its numerous highly differentiated species, 
is divided into six sub-families, on the basis of the simplicity or complexity of the 
jugum. These are the Rhynchospirinae, Hindellinae, Athyrinae, Diplospirinae, Koninck- 
ininae, and Meristellinae. 
Rhynchospira, Hall (Fig. 578, C, D). Rostrate, radially plicate shells with 
a short curved hinge-line; apex truncated by a circular pedicle opening. 
Dorsal hinge-plate like that of Trematospira. Spirals with from six to nine 
volutions. Jugum V-shaped, expanding apically and terminating posteriorly 
in an oblique edge. Shell structure punctate. Devonian to Lower Carboni- 
ferous ; North America and Europe. 
Homoeospira, Hall and Clarke. Like the last, but differs in having a linear 
cardinal process sepayr- 
ating the crural plates. 
Jugum not apically 
expanded, but termi- 
nating in an acute 
stem. Silurian ; North 
America. 
Ptychospira, Hall 
and Clarke (Fig. 578, 
A, B). Like Rhyncho- 
spira, but with a few 
angular plications. The A, B, Ptychospira ferita, von Buch. Devonian; Gerolstein, Eifel. qj. 
jugum has along simple C, D, Rhynchospira Salteri, David. Silurian ; Wenlock, Shropshire. ¢ In- 
process passing out- Hen eon ne D, Median vertical section through both valves, 3/1 
ward between the coils 
to near the inner surface of the ventral valve. Devonian to Lower Carbon- 
iferous ; North America and Germany. 
Retzia, King (Trigeria, Bayle). Resembling Rhynchospira externally. The 
ventral umbonal cavity has a split tube. The single process of the jugum 
terminally forked. Devonian ; Europe. 

Fic. 578. 
This genus formerly contained all shells having a Retzioid exterior. At present, 
however, but a single species is admitted. 
Hustedia, Hall and Clarke. Externally like Eumetria, but with coarse 
plications, and internally with a split tube, as in Retzia. Spirals and jugum 
similar to that of Hwmetria, but with fimbria and spinules respectively. 
Upper Carboniferous ; America, Europe, India, and China. 
Trematospira, Hall. Transverse Ehynchospirae. Silurian and Devonian ; 
North America. 
VOL. I Z 
