288 



THE VOYAGE OF H.M.S. CHALLENGER. 



section Cenoprunida. All other families possess internal medullary shells, and so repre- 

 sent the section Coccoprunida. 



Another character, which can be employed in the arrangement of the seven 

 subfamilies in some larger groups, is the presence or absence of ring-like constrictions, 

 by which the cortical shell is divided into chambers. I. The Monoprunida comprise all 

 forms without any constriction, of which the EUipsida are without a medullary shell, the 

 Druppulida with one or two medullary shells, and the Spongurida with a spongy 

 cortical shell. II. The Dyoprunida contain all forms with a cortical twin shell, or with 

 two chambers separated by one equatorial constriction, of which the Artiscida are 

 without a medullary shell and the Cyphinida have one or two medullary shells. 

 III. The Polyprunida comprise all forms with several (three or more) constrictions, 

 which separate four or more chambers, of which the Panartida have three constrictions 

 and four chambers, and the Zygartida five or more constrictions and six or more chambers. 



The Central Capsule of the Prunoidea is originally ellijjsoidal (monaxiaJ), and 

 preserves this form in the greater part of the genera. In some groups, where the axis of 

 the ellipsoid is much prolonged, it passes over to the cylindrical form (with hemispherical 

 vaultings on both poles), as in Spongurus and Spongocore, in many Panartida and 

 Zygartida. Yery often the ellipsoidal or cylindrical capsule gets annular transverse 

 constrictions, corresponding to those of the enveloping cortical shell (one single, equa- 

 torial stricture in the Artiscida and Cyphinida, three strictures in the Panartida, five or 

 more in the Zygartida). In the Cenoprunida (EUipsida and Artiscida, also in Spongel- 

 lipsis) the central capsule lies freely in the cavity of the cortical shell, separated from its 

 inner surface by the jelly-envelope ; in all other groups it contains a part of the skeleton, 

 the meduUary shell and the beams which connect it with the enveloping cortical shell. 



Synopsis of the Families of Prunoidea. 



A. MONOPEUNIDA. 



Shell without transverse stricture. 



B. DYOPEUNIDA. 



Shell bilocular, divided by an equa- 

 torial stricture into two communi- 

 cating hemi-ellipsoidal shells. 



C. POLYPKUNIDA. 



Shell multilocular, divided by three or 

 more parallel transverse strictures into 

 four or more serial camerae. 



a. Shell simple, latticed (not spongy), with- 

 out enclosed internal shells, . .1. Ellipsida. 



I). Shell composed of two or more con- 

 centric latticed shells (not sjiongy), . 2. DRirpULiDA. 



c. Shell partially or wholly composed of 



an irregular spongy framework, . . .3. Spongcrida. 



\ d. Shell simple, without enclosed internal 



I shells, 4. Artiscida. 



.; 



e. Shell composed of two or more con- 

 centric shells, 5. Cyphinida. 



' /. Shell with tliree parallel strictures and 



therefore four cameras, . . . G. Panartida. 



ij. Shell with five or more parallel strictures 



and therefore six or more cameroe, . 7. Zygartida. 



