228 



MANUAL OF THE MOLLUSCA. 



Fi^. 143. Longiludinal; Transverse section. 



The relations of the animal to the shell, in such a species as P. Knightii 

 can only be inferred by comparison ^^^th other species in which the internal ! 

 plates are less developed, and with other genera, such as Cyriia and Camaro- 

 pkoria. In fig. 143, the small central chamber {v) must have been occupied i 

 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 I 

 plates converge, and in Camaroplioria the muscular impressions occupy the < 

 same position as in Rhi/ticJionella. 



Atrypa, Dalman. ■ 



Syn. Cleiothyris, Phillips. Spirigerina, D'Orb.* Hipparionyx, Vanusem. ;i 

 Type, A. reticularis, PI. XV. fig. 17- Pigs. 144, 145. I 



\ 



Fig. \4^:, Dorsal 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 squamosa 

 lines of growth ; dorsal valve gibbose ; ventral depressed in front ; beak 



* The term Ainjpa {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! 



