800 Report oF THE StatTE GEOLOGIST. 
and then widen again, attaining their greatest width where 
they unite. From their line of union there is a short, acute 
and simple process extended horizontally 
backward. The attitude of the jugum 
is erect, at its base extending slightly 
backward and then curving broadly 
forward and upward; in height, it 
reaches rather more than half-way across 
Fig. 370.— Jugum of Trematos- the bases of the spiral cones. The cones 
Sah aa Rene are situated as in allied genera and the 
ribbon, in mature shells, makes nine or ten volutions. Muscular 
impressions indistinct. Shell substance punctate. 
Type, Zrematospira multistriata, Hall. Lower Helderberg 
group. 
Distribution. Upper Silurian — Devonian. 
Parazyga, Hall. 1893. 
(Plate 88, figs. 18-20.) 
Shells similar in general external aspect to TreMmArTospira. 
The surface markings consist of numerous fine, rounded, simple 
ribs, extending alike over median fold and sinus, and these are 
covered with exceedingly fine, short, hair-like spines, which are 
usually broken off, leaving only their bases. 
The umbo of the pedicle-valve is closely incurved and the del- 
tarium (or coalesced deltidial plates) which is entirely concealed 
by the umbo of the opposite valve, is usually lost. The apical 
portion of the umbonal cavity bears an introverted lamella which 
forms an incomplete tube like that in Rerzrta, Husrepra, etc., but 
of no great extent. The teeth are as in Tremarosprra and are 
supported by stout plates. The muscular area is short, rather 
well defined, and is divided into a broad central adductor impres- 
sion, along the lateral margins of which lie two flabellate 
diductor scars. 
The hinge-plate is very narrow, and is composed of two ver- 
tical supports which have their origin on the downward umbonal 
slope of the shell. These supports are widely separated at their 
bases and enclose the marginal dental sockets; their anterior 
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