54 THE 3IICI10SC0PE. Apr., 



girdle what is meant is that the shell loses its convexity 

 and becomes flattened at the margin. Imagine a bi-con- 

 vex lens turned edgewise and a ring of thin glass encir- 

 cling it in the equatorial plane and you will understand 

 what is meant by the "hyaline equatorial girdle" of 

 Periphgena. If on the contrary the margin of the pha- 

 coid shell, when only one surface is presented, is surroun- 

 ded by concentric chambeied girdles or rings it belongs 

 to the Coccodiscida. "Each of these chambered gir- 

 dles," says Haeckel, is composed of a circular ring in 

 the equatorial plane, a variable number of radial beams 

 dividing it into incomplete chambers, andtw^ porous 

 cover-plates or " sieve-plates," covering the upper and 

 lower face of the disk. These sieve-plates may be 

 regarded as incomplete lenticular cortical shells, which 

 are only developed in the peripheral part of the disk, 

 whilst their central part is represented by the only com- 

 plete cortical shell, ihe "phacoid shell." The general 

 appearance is that of fig. 15. Here again the presence 

 or absence of spines or arms or membrane will serve to 

 distinguish the genus. By chambered arms are meant 

 such projections as the four club-shaped portions of fig. 

 20, and by membrane the fine spongy framework be- 

 tween the arms in the same figure, the technical term for 

 which membrane is " patagium." 



If your form is not like fig. 14 but like fig. 16, it be- 

 longs to div. B. In this division there is no "phacoid " 

 shell, but a flat discoidal shell. There are two families 

 in this division, the Porodiscida and the Spongodiscida. 

 In the former, instead of a phacoid shell, there is a 

 small simple central chamber surrounded by a number 

 of small latticed chambers of nearly the same size and 

 form (Haeckel). The surface of the disk on the two flat 

 sides is covered by a porous sieve-plate.-"^ In the first 

 section there are neither spines nor arms nor any wide 



^Challenger Report. 



