378 



MA-NXTAI OF THE MOLLTTSCA. 



ir P. ? hrevirostris (Devonian, Newton) the dorsal valve tas 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. Knightii 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 {d s) 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, Yanuxem. 



Type, A. reticularis, PL XY., Fig. 21. Figs. 180, 181. 



Fiff. 180. Dorsal valve. 



Fig. 181, Ventral valve ; interiors. 



p, hinge-plate; a, impressions of adductor muscle ; c, cardinal muscle ; p, pedicle 

 muscle ; o, 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 lamellae ; 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 

 addition to Spirifera, Spii-igera, and Spirifezina 



