286 THE VOYAGE OF H.M.S. CHALLENGER. 



A tliii-d family of P r u n o i d e a, closely allied to the two preceding families, is the 

 Spongiirida, iu which we include all Prunoidea with an ellipsoidal or cylindrical, 

 unjointed shell, in which the lattice-work of the cortical shell is transformed into an 

 irregular, sUiceous framework. In the simplest form, Spongellipsis, the simple lattice- 

 shell of Cenellipsis is substituted by an external spongy envelope. In other cases 

 [Spongurus and allied genera) the whoje cavity of this external spongy shell is distended 

 with a fine spongy framework. The subfamily of Spongodruppida is distinguished 

 by the possession of a simple or double latticed medullary shell ; this lies in the midst 

 of the central capsule, and is connected hj radial beams (perforating its membrane) with 

 the enveloping spongy cortical shell. The surface of the latter may bear either radial 

 spines, or two opposite strong polar spines, at the poles of the main axis (PI. 17, fig. 12). 



Closely allied to the EUipsida and Druppulida are two other families of the 

 Prunoidea, the Artiscida and Cyphinida, which difier from the former by a circular 

 constriction in the equatorial plane of the ellipsoidal shell ; and in this way assume a 

 characteristic twin form, like a figure of eight. In the Artiscida the shell is simple (as 

 in the EUipsida), whereas in the Cyphinida it is composed of two or more concentric 

 shells (as in the Drupiaulida). The simplest form of the Artiscida is Artiscus 

 (PI. 39, fig. 9), difi'eriug from Cenellipsis in the ring-shaped, equatorial constriction. 

 In other Ai'tiseida polar appendages are developed on both poles of the main axis, either 

 in the form of solid, strong spines [Stylartus), or hollow fenestrated tubes (Cannartus, 

 PI. 39, fig. 10). 



The family Cyphinida differs from the Druppulida in the equatorial constriction of 

 the shell, and from the Artiscida in the presence of two or more concentric shells. 

 One or two of these concentric fenestrated shells are enclosed in the central capsule 

 (and therefore may be called " medullary shells"); the others (one or two, rarely more) lie 

 outside of the central capsule (therefore "cortical shells"). The internal "medullary 

 shells " are always spherical or somewhat lenticular, compressed from both sides ; the 

 external " cortical shells " have constantly a ring-like constriction in the equatorial plane, 

 and " twin-shells " are therefore like a figure of eight. The simplest form of this 

 subfamily is Cyp>}ianta, composed of a simple medullary shell and a simple cortical shell, 

 the two being connected in the ecjuatorial plane by radial beams. In Cyphonium (PI. 39, 

 fig. 12) the medullary shell is doubled, and in Cypassis (PI. 39, fig. 13) the cortical shell 

 likewise. On both poles of the main axis strong spines are often developed {Cyphinus, 

 PI. 39, fig. 14), or hollow fenestrated tubes (Cannartidium, PL 39, figs. IG— 19). 



The equatorial constriction of the ellipsoidal shell, which characterises the 

 Artiscida and Cyphinida, is repeated or multiplied in the two foUowiug families, 

 in the Panartida and Zygartida ; in the former we find three ring-like strictures, 

 in the latter five or more (lying in parallel transverse planes) ; therefore the fenes- 

 trated shell is composed in the one instance of four chambers, in the other of six or 



