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 P h se o c o n c h i a) 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 oj)eu 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 ^dsible, 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 parapylte, 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 phaeodella (PI. 123, figs. 8, 9) ; but usually 



the phaeodium 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.eodaria smaller or somewhat different in 

 shape from the venti'al valve, and this difference is often very stinking (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 Coelodendrida ; in these latter as well as in the 

 Ccelographida, 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 phaeodium ; 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- 



