GENUS CHELONOBIA. 385 



itself does not descend to the basal membrane. In C. tes- 

 tudinaria, the inner lamina is of great thickness ; but in the 

 section, (fig. 1) owing to its having been taken high up, the 

 inner lamina, (6), is not distinct from the shelly matter depo- 

 sited between the septa. In C. caretta, the line of separation 

 between the inner and outer laminae can in no part be dis- 

 tinguished, owing to the interspaces between the septa 

 having been solidly filled up, close down to the basis. 



Sheath. — The layer of shell surrounding the internal 

 cavity (ece, fig. 4, PL 14), and extending down to the basal 

 membrane, I have called the sheath, owing to its being 

 distinctly continuous with the innermost layer in the upper 

 part of the shell, to which the opercular membrane is at- 

 tached : this can be best seen by examining the alae in the 

 separated compartments of C. patida. The sheath is not 

 only remarkable from thus descending to the basal mem- 

 brane, but in C. testudinaria and patida from its lower edge 

 being perforated by arched channels (under c, in fig. 4), 

 allowing thick ribbons of corium to reach the interspaces 

 between the radiating septa. There is one central arch or 

 channel in the middle of the lower part of the sheath of each 

 compartment, and one on each line of suture, the sheath 

 being a little hollowed out on both sides of the sutures. As 

 the rostrum, as far as its internal structure is concerned, 

 consists of three compartments, we have altogether in the 

 shell eight compartments and eight sutures, and conse- 

 quently altogether sixteen arches through the lower part of 

 the sheath, allowing sixteen thick ribbons of corium to 

 penetrate the parietes, and thus likewise reach the radii. 

 There is, however, sometimes a little variation in the 

 number of these arched channels. The upper part of the 

 sheath is transversely marked by zones of growth, to the 

 lower one of which the opercular membrane is attached. 

 The line of attachment is not low down the sheath. 



Radii and Ala. — The radii, when the compartments are 

 disarticulated, present a remarkable structure, from appear- 

 ing to consist of a distinct inner and outer portion. The 

 radius normally consists of an inner and outer lamina, 

 united by septa parallel to the basis ; but here the inner 

 portion is formed by a central ridge {a a, fig. 5, PL 14), send- 



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