228 



MANUAL OF THE MOLLUSCA. 



Fig. 143. Longitudinal; Transverse section. 



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, a*hd with other genera, such as Cyrtia and Camaro- 

 phoria. In fig. 143, 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 Porambo- 

 nites 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. 



ATRTPA, Dalman. 



Syn. Cleiothyris, Phillips. Spirigerina, D'Orb.* Hipparionyx, Vanuxem. 

 Type, A. reticularis, PL XV. fig. 17. Pigs. 144, 145. 



Fig. I'M, Dor sal valve. Fig. 145, 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 



* 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, Spirigera, and Spiriferina ! 



