1712 THE VOYAGE OF H.M.S. CHALLENGER. 



spherical lattice-shell (Castanellida), by the halving of this latter, or its splitting into 

 two hemispherical valves ; the former hypothesis is more probable than the latter. 



The two valves of the lattice-shell (Pis. 123-126) must in the Concharida (as in 

 all other Phseoconchia) be distinguished as dorsal and ventral, and may therefore 

 be compared with the two valves of the Brachiopoda, not with those of the Lamelli- 

 branchia. This important morphological distinction is expressed by the constant 

 position of the central capsule within the shell-cavity. The capsule always exhibits 

 the character of the " TRIPYLEA " and has three tubular openings, placed in the frontal 

 or lateral plane of the unicellular body. In the same plane lies the open frontal 

 fissure between the two valves, and the three openings are so disposed in it that the 

 large anterior main-opening (or the astropyle) is placed on the oral pole of the main 

 axis, whilst the two accessory small lateral openings or parapylse are placed on both 

 sides of the aboral pole, at the right and left. Therefore in a dorsal or ventral 

 view all three openings are visible (PI. 123, figs. 1, 8a) ; in the usual lateral view, 

 however, from the right or left side, only two openings are visible, the astropyle on 

 the anterior, and one parapyle (right or left) near the posterior pole of the main axis 

 (PI. 123, figs. 8, 9; PI. 124, figs. 6, 10). The posterior view (from the aboral pole) 

 shows the two parapylse, at right and left ; in the anterior view (from the oral pole) 

 the astropyle may be visible, but usually it is completely hidden in the dark 

 voluminous phaeodium. This latter envelops sometimes nearly the whole capsule as an 

 opaque conglomeration of green or brown phseodella (PI. 123, figs. 8, 9); but usually 

 the phgeodium fills up the anterior (oral) half of the shell-cavity, whilst the capsule 

 occupies the posterior (aboral) half (PL 124, figs. 6, 10). 



The dorsal shell-valve is in almost all PH^EODABIA smaller or somewhat different in 

 shape from the ventral valve, and this difference is often very striking (PI. 124, 

 figs. 3-16); but in a few species both valves are so similar, that I could not discover 

 any certain difference. This equality of the two valves occurs mainly in those 

 Concharida which pass over into the Ccelodendrida ; in these latter as well as in the 

 Coelographida, both valves are usually equal in size and form. Whilst the main axis 

 (or the longitudinal axis of the body) in the two latter families of Phseoconchia 

 seems to be normally vertical (in the living and -freely floating body), in the living 

 Concharida it is probably horizontal, so that the larger and heavier ventral valve lies 

 below the smaller and lighter dorsal valve. 



The geometrical fundamental form of the body is therefore in the Concharida 

 dipleural or bilaterally symmetrical, and we distinguish in it the same three dimensive 

 axis, as in all other dipleural forms. On the anterior or oral pole of the main axis (or 

 longitudinal axis) lies the mouth of the shell, and behind it the phseodium ; on the 

 opposite posterior or aboral pole lies the hinge of the shell (comparable to the shell- 

 hinge of the Brachiopoda) and in front of it the central capsule. The sagittal (or dorso- 



