378 MANUAL OF THE MOLLUSCA. 
in P. ? brevirostris (Devonian, Newton) the dorsal valve has a 
long trough-like process supported by a single low septum. 
Fossil, 52 species. Upper Silurian— Devonian. Arctic 
America, United States, Europe. 
The relations of the animal to the shell in such a species as 
P. Knightti can only be inferred by comparison with other 
species in which the internal plates are less developed, and with 
other genera, such as Cyrtia and Camarophoria. In Fig. 179, 
the small central chamber (v) must have been occupied by the 
digestive organs, the large lateral spaces (ds) by the spiral 
arms; it is doubtful whether any muscles were attached to these 
plates; in Porambonites the adductor impression is situated 
beyond the point to which the dental plates converge, and in 
Camarophoria the muscular impressions occupy the same position 
as In Rhynchonella. 
ATRYPA, Dalman. 
Synonyms, Cleiothyris, Phillips. Spirigerina, D’Orbigny.* 
Hipparionyx, Vanuxem. 
Type, A. reticularis, Pl. XV., Fig. 21. Figs. 180, 181. 
Fiz. 180. Dorsal valve. Fig. 181. Ventral valve ; interiors. 
p, hinge-plate; a, impressions of adductor muscle; c, cardinal muscle; p, pedicle 
i muscle; 0, ovarian sinus; d; deltidium. 
Shell impunctate; oval, usually plaited and ornamented with 
squamose lines of growth; dorsal valve gibbose; ventral 
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 
* The term Atrypa, (a, without, trupa, foramen) is objectionable, like all Dalman’s 
names; but M. D’Orbigny has made no improvement by proposing Spirigerina, in 
uddition to Spirifera, Spirigera, and Spiriferina 
