416 BALAMD.E. 



diameter; this individual was one of the more convex varieties, and 

 yet its entire thickness, from top to base, was only 9 of an inch. 



Operculum. — Having described, under the genus, the general struc^ 

 ture of the opercular valve, I will here only add a few details on their 

 shape. They are small compared with the whole extent of the opercular 

 membrane. The scuta (PI. 15, fig. 2 6, scutum to the left) stand near 

 each other at the rostral end of the aperture leading into the sack, with 

 their two rostral ends united by the yellowish-brown, longitudinally 

 plicated, horny membrane, described under the genus. The terga 

 (fig. 2 b) stand a little apart from the scuta, on the sides of nearly the 

 middle of the sack-aperture. The lips are protuberant and moderately 

 developed all round. The scuta are elongated, and a little curved ; in- 

 cluding the upper imbedded portion, they are almost sub-triangular: 

 but the under growing surface is much elongated, nearly flat, with the two 

 ends of nearly the same width, truncated and rounded. The Tergum, in 

 rather small specimens is, if the upper imbedded portion be included, 

 sub-triangular, with the growing surface oval, and between one third 

 and one half of the length of the scutum ; but in large specimens, the 

 tergum becomes style-formed, lying parallel to the tergal margin of the 

 scutum, with the growing surface proportionally much smaller, and not 

 above one fifth of the length of the growing surface of the scutum. Hence 

 we see some tendency in the tergum to become rudimentary, as it is in 

 the next species. The brown, horny, plicated substance in which the 

 terga are imbedded, extends considerably beyond the valves themselves. 



Structure of the Shell and Radii. — I have already so fully discussed 

 this subject, that I will here only enumerate the points in which this 

 species differs from C. diadema and regince : — Firstly, the more symme- 

 trically folded walls (PI. 16, fig. 5), new folds arising on both sides of 

 all six sutures. Secondly, the inner ends of the folded walls, which 

 surround the internal cavity, being almost square, but with their angles 

 rounded ; their inner ends descend some little way beneath the basal 

 edge of the sheath, as low, or lower, than the circumference of the shell. 

 Thirdly, the external lines of junction between the transverse terminal 

 loops being smooth or not serrated. Fourthly, these loops being flatter 

 towards the outside, and being iuternally filled by septa (PI. 15, 

 fig. 2 a) ; tubes being thus formed larger than the proper parietal pore*. 

 Sixthly, the sutural edges of the compound radii (PI. 16, fig. 3) being 

 very much broader in the inner and upper part of each compartment 

 than in the outer and lower part; for in the inner and upper part they 

 stretch from the outside of the shell to the sheath, so that the alae 

 rest on them, and hence no large open cavity is left between the 

 radii and alae; in the inner and upper part, also, the radii extend 

 down nearly to the basal edges of the folded walls. The septa, of 

 which the radii are formed, stand further apart than in C. diadema 

 and regince. Seventhly, the alae are only l-5th or l-6th of the thickness 

 of the radii, whereas in the two other species they are very much 

 thicker, being nearly as thick as the radii : the alae are also here (fig. 3, 

 a') squarer than in those two species, that is, their basal margins are 

 not so short compared with their upper margins ; their edges present, 

 also, a slightly different structure. Eighthly, the basal edge of the 



