MANUAL OF THE MOLLUSCA. 
RHYNCHOSPIRA, Hall, 1859. 
Hiymology, pvyxoc, a beak, and spira; in allusion to its 
similarity in form to Rhynchonella, and haying internal spires. 
Type, Waldheimia formosa, Hall. 
Shell somewhat similar to Rhynchenella, but usually more 
symmetrically rounded, and with less distinct mesial sinuosities ; 
and in these characters they resemble Waldheimia. 
Valves articulated by teeth and sockets, similar to those of 
Nucleospira; the crura supporting two conical spires. The 
cardinal. process of the dorsal valve is a broad emarginate plate ; 
beak of the ventral valve largely perforated. Surface plicated 
or striated. 
Distribution, 7 species. Silurian—Deyonian. United States, 
Russia. 
ArryPa (see p. 378). 
The internal appendages of Atrypa reticularis (see Fig. 24) 
- consist of a pair of spiral cones, placed side by side, with their 
apices directed towards the cavity of the dorsal valve; the 
lamellee have their origin on the socket-walls, and run parallel 
y 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 
| f=| shell. Thelooparisesfrom the posterior 
f=) portion of the first vyolutions of the 
=) spires, and curves gently forward 
and upward; the central or elevated 
portion is situated between and be- 
hind the cones, and forms a more or 
less abrupt curve, or is prolonged - 
into a point directed towards the 
dorsal yalye. The existence and form of this loop haye been | 
ascertained in several different varieties of A. reticularis, as 
well as in A. spinosa, Hall.” —( Whitfield. ) 
Sub-genus, ZyGospira, Hall, 1862. 
Synonym, Stenocisma, Conrad, 1847. 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 haying 
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. 
58 
Fig. 24, 
