1732 THE VOYAGE OF H.M.S. CHALLENGER. 



of the two external mantle-valves, opposed to one another in the circle of the equator, 

 seem usually to catch one into another in the same way as the corresponding man tie - 

 valves of the Coeloplegmida are loosely connected (PI. 128, figs. 1, 7). A true con- 

 crescence between the two valves seems never to take place. 



The two subfamilies of Ccelodendrida therefore exactly correspond to the two sub- 

 families of the following family, the Coelographida. The Ccelodorida and Coelotholida 

 form in a similar way a thicket, by dichotomous ramification of the hollow tubes, all the 

 branches of which remain free. The Coelodrymida and Coeloplegmida, on the other hand, 

 form an outer lattice-mantle by anastomosing branches. The latter two subfamilies, of 

 course, have been derived correspondingly from the two former, and the common ancestral 

 form of all four is probably Ccelodoras, derived from the Concharida. 



Though the two corresponding subfamilies in both groups are very similar, they are, 

 however, separated by important hereditary characters. All Ccelodendrida (the Ccelodorida. 

 without a mantle as well as the Ccelodrymida with a mantle) possess no rhinocanna and 

 no frenula on the galea, and they never develop prominent verticillate styles ; the 

 surface of their calymma is probably always spherical or subspherical. All Ccelographida, 

 however (the Ccelotholida without a mantle as well as the Cceloplegmida with a mantle), 

 possess a rhinocanna and frenula on the galea, and always develop prominent verticillate 

 styles ; the surface of their calymma is probably alwaj^s symmetrically polyhedral. 



The superficial armature of the skeleton in the Coelodendrida is rather simple, and by 

 no means so manifold and differentiated as in the more highly developed Coelographida. 

 The thin terminal branches of the hollow tubes are in the Coelodorida closed at the distal 

 end, and armed with a variable number of short teeth (PI. 121, fig. 2), or with a spinulate 

 terminal knob, or a corona of recurved hooks (ibid., figs. 5-7). In the Coelodrymida, 

 however, where the distal ends of the branches by anastomosing form the lattice- 

 mantle, the spherical surface of this latter is armed with numerous thin spathilla3 or 

 radial bristles (often zig-zag or spinulate), and each bristle usually bears at the distal 

 end a small anchor with two, three, or four recurved teeth ; the outer convex edge of 

 these teeth is usually smooth, the inner concave edge denticulate. All these ramules and 

 branches of the tubes (also the thinnest terminal threads) are hollow, and filled up by 



J ell 7- 



The central capsule of the Ccelodendrida does not lie outside the two central valves (as I 



supposed in my first description, in 1862, being deceived by the dark enveloping phseodium, 

 Monogr. d. Eadiol., Taf. xxxii. fig. 1), but it is enclosed between the two valves, as in the 

 preceding and the following family. The first accurate description of it was given by 

 Eichard Hertwig in 1879 (loc. cit., p. 95, Taf. x. fig. 3). Its constant position between 

 the two lattice- valves (dorsal and ventral) is such, that its three openings lie in the frontal 

 plane, in the open fissure between the valves. The astropyle or the main-opening, with the 

 radiate operculum and the tubular proboscis arising from it, lies on the anterior (or oral) 



