ATRYPIDAs. 319 
depressed in front; beak small, often closely incurved ; foramen 
round, sometimes completed by a deltidium, often concealed ; 
dorsal valve with a divided hinge-plate, supporting two broad 
spirally coiled lamelle; spires vertical, closely appressed, and 
directed towards the centre of the valve; teeth and impressions 
like Rhynchonella. 
The shells of this genus differ from Rhynchonella chiefly in 
the calcification of the oral supports, a character of uncertain 
value. 
The internal appendages of Atrypa reticularis consist of a 
pair of spiral cones, placed side by side, with their apices directed 
towards the cavity of the dorsal valve; the lamellz have their 
origin on the socket-walls, and run parallel with the inner margin 
of the valve. “ The spiral cones are connected by an entire and 
continuous loop, which is confined to the rostral part of the shell. 
The loop arises from the posterior portion of the first volutions 
of the spires, and curves gently forward and upward ; the central 
or elevated portion is situated between and behind the cones, 
and forms a more or less abrupt curve, or is prolonged into a 
point directed towards the dorsal valve. The existence and form 
of this loop have been ascertained in several different varieties 
of A. reticularis, as well as in A. spinosa, Hall.” —WHITFIELD. 
CELOSPIRA, Hall, 1863. Shell fibrous, concavo-convex; spiral 
lamelle with their bases turned towards the ventral, their points 
towards the dorsal valve. A. camilla, Hall (exxxvii, 92). 
STENOSCHISMA, Hall, 1847. (Zygospira, Hall, 1862.) Spiral 
cones connected by an entire and continuous loop in a very 
similar manner to that shown to exist in Atrypa reticularis ; but 
the loop having its connection with the spiral lamelle at a point 
relatively more distant from their origin on the hinge-plate, and 
passing over or in front of the spires. A. modesta, Say. Silu- 
rian’: U2 S: 
GuasstA, Davidson, 1881. 
Distr.—3 sp. Silurian; England. G. Whidbornei, Davidson 
(cxxxvii, 93). 
Principal lamelle, forming the first coils of the spirals, con- 
nected at a short distance from the attachment to the hinge-plate 
by a ribbon-shaped lamella or loop; this loop, commencing on 
each side from the principal lamellae, converges downwards in 
the shape of the letter V ; principal coils of the spirals directly 
face the lateral margins; the ends of the spirals meet each other 
in the centre of the shell; each spiral consists of four coils. 
ANAZzYGA, Davidson, 1882. 
Distr.—A. recurvirostra, Hall (exxxvii, 94). Trenton lime- 
stone, Canada. 
Shell small, longitudinally oval, radiately striated ; position 
