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



triangiilar meshes. On the other hand, the vohiminous shell of the Aulosphserida 

 (Pis. 109-111), and of the Cannosphserida (PL 112), is characterised by a very 

 peculiar system of joints ; it is composed of numerous separate cylindrical tubes, which 

 are placed tangeutially and united at the nodes by stellate partitions or astral septa. 

 The Cannosphserida possess further a simple central Cyrtoid shell, connected w4th the 

 outer jointed shell by hollow radial trabeculse. Since many Aulosphserida possess 

 rudiments of such centripetal trabecules it is possible that these latter have been 

 derived from the former by the loss of the central Cyrtoid shell ; the formation of this 

 monaxon shell perhaps indicates descent from the Phseogromia (CastaneUida). 



198. Phceogromia ivith a Cyrtoid Skeleton. — That order of the Ph.eodaria which 

 we designate Phseogromia, contains many very different forms, all agreeing in the 

 possession of a Cyrtoid skeleton, or a monaxon lattice-shell, which has a large aperture 

 at one pole of its vertical main axis (§ 123). This Cyrtoid skeleton is sometimes ovoid 

 or conical, sometimes lentiform or helmet-shaped, sometimes polyhedral or almost 

 spherical. Although the principle of its structure is simple and often very like that of 

 the Monocyrtida among the Nassellaria, yet the structure of the ^vall and of the 

 apophyses is so different in the various groups of the Phseogromia, that the order 

 is probably polyphyletic, and its CjTtoid shells have arisen independently of each other. 

 Only in the CastaneUida (PL 113) does the sheU-wall usuaUy consist of simple lattice- 

 work ; in the ChaUengerida, on the other hand (PL 99), it has an extremely fine 

 Diatom -like structure;, in the Medusettida (Pis. 118-128) a peculiar alveolar structure, 

 and in the Circoporida (Pis. 1 14-1 17) and Tuscarorida (PL 100) it possesses a charac- 

 teristic porceUanous constitution (with tangential spicules in a porous cement-mass) ; 

 in the latter of these groups the surface is smooth, in the former peculiarly talnilate ; 

 the two families have also different stem-forms. 



199. PhcBOCOnchia with a Conchoid Shell. — The order Pheeoconchia (Pis. 121- 

 1 28) is separated not only from all other Ph jeodaria, but also from all other Eadiolaria, 

 by the possession of a bivalved sheU resembling that of a Lamellibranch ; the two valves 

 of this Conchoid skeleton are to be interpreted as dorsal and ventral (§ 128). Probably 

 these bivalved shells are independent products, but possibly they may have been 

 formed by the bisection of a simple spherical lattice-sheU ; in the former case the 

 Phaeoconchia would be directly descended from the Phseodinida, in the latter from 

 the CastaneUida. The three famUies which we have distinguished among the P h se o- 

 c n c h i a, probably constitute a connected stem, the most primitive group of which are 

 the Concharida (Pis. 123-125). From these the Coelodendrida (Pis. 121, 122) 

 have next arisen by the formation of a " galea " upon the apex of each valve, and the 

 growth of hoUow tubes from this helmet-like structure. Finally, the Coelographida 



