406 balanim;. 



running along each suture, like those which run along the 

 bases of the spoke-like folded walls. This difference in the 

 origin of the ribbons of corium, occupying the interspaces 

 between the plates and the pores in the proper radii, shows 

 the essential difference between the latter and the thick inner 

 portions of the compound radii. In C. balcenaris (fig. 3), the 

 compound radius extends from the outside of the shell to the 

 sheath and to near the basal edges of the folded walls : in C. 

 diadema (fig. 2), it does not extend so far inwards and down- 

 wards ; and in C. regince (fig. 4), even still less so. It ensues 

 from this circumstance that when, in these two latter species, 

 a transverse section is made across the middle of the shell, a 

 large chamber (v, fig. 7), occupied by the ovaria, is found on 

 each line of suture (ss), separating the radii (d) and alas (a). 



The Alce'xw C. diadema (section, PI. 16, fig. 7, a) and C. 

 regince, are of remarkable thickness, nearly equal to that of 

 the radii, and in C. balcenaris, of considerable thickness ; 

 this is evidently to give strength to the shell, which is 

 weakest along the lines of suture, notwithstanding that the 

 radii have been specially thickened. The edge of each ala 

 presents a miniature resemblance of the edge of the radius, 

 namely, a central ridge sending off on both sides sinuous 

 plates, themselves denticulated. In C. balcenaris^ the ala 

 rests almost entirely on the inner surface of the compound 

 radius ; but in the other two species, in which, as already 

 stated, the radii and alas are separated by chambers, the ala 

 rests on a plate (c" in fig. 1, 4, and 7), which extends from 

 the top of the radius to the bottom of the sheath, narrowing 

 downwards (c\ fig. 1), and is a specially developed portion 

 of the sheath for the radius to rest on. In C. diadema and 

 regince, the sides of the folded walls, at the ends of the 

 chambers (v, in section, fig. 7), are strengthened by the 

 deposition of layers of shell in connexion with the sheath. 

 The sheath extends close down to the basal membrane 

 in C. balcenaris, and does not project freely : in the other 

 two species it depends freely, but does not run quite so low 

 down. I have only further to remark, that the sutures, 

 though very strongly united, are not calcified together ; for 

 they easily separate after the action of caustic potash. 



The Basis (PI. 28, fig. 1 a — 1 c) is membranous and is 



