1018 THE VOYAGE OF H.M.S. CHALLENGER. 



as in that family ; it is therefore probable that the majority of the S p y r o i d e a (if not 

 the whole group) have been derived directly from the Semantida by further development 

 of network from the fenestrated ring of Clathrocircus. Whilst in this latter Semantid 

 the two lateral sides of the central capsule remain naked, in the Spyroidea they 

 become enclosed and covered by lattice-work, which arises on the right and left from 

 the ring. In some forms, however, as in the Ceratospyris OAiuminata of Hertwig, and in 

 my Perispyrida, the frontal ring also of the Coronida and the mitral ring of the Tympanida 

 appear as the initial parts of a complete latticed cephalis. 



The basal plate of the cej)halis, or the " cortinar plate," the " Basal-Scheibe " of 

 Biitschli, has constantly, according to this author, two pairs of pores, an anterior smaller 

 pair of jugular pores (the pores I of Biitschli) and a posterior larger pair of cardinal 

 pores (the pores II in his description) (compare PI. 95, figs. 3, 6). This applies to 

 the majority of Sjjyroidea, but by no means to the whole group. We find basal 

 plates with two pores only (PL 95, fig. 1), with three pores (figs. 2, 5), with, six pores 

 (figs. 4, 7), with nine pores (PI. 87, fig- 2), &c. The forms with two basal pores may be 

 derived directly from Semantis (PI. 92, figs. 1, 2); the common forms with four pores 

 from Semantrum (figs. 3-5) ; the rarer forms with six pores from Semantidium (figs. 6, 7); 

 and the forms with three pores from Cortiniscus, &c. In this as well as in other 

 respects the variety of difi'erent forms and of developmental variations is far greater than 

 Biitschli (1882, loc. cit.) supposed. The bars between the pores of the basal plate 

 possess the same value and the same names as in the Semantida (compare above, p. 954). 



The coryphal plate of the cephalis, its ujDper or apical lattice-jDlate, does not exhibit such 

 important difi'erences as the opposite basal plate, is far less variable, and is usually pierced 

 by numerous, smaller pores. Some larger pores lie, often in pairs, on the right and left 

 side of the sagittal constriction, and have in some groups a regular form and disposition. 

 Either in the middle of the coryphal plate, or (usually) nearer to its dorsal margin, 

 there arises in the majority of S p y r o i d e a an apical horn, directed either vertically or 

 more or less obliquely backwards. In many Spyroidea three horns are developed in 

 the coryphal plate, the odd middle apical horn and two paii-ed frontal or lateral horns 

 arising on each side of the latter and directed more forwards (PL 84, figs. 9—12). 

 Sometimes the apical horn disappears, while the two frontal horns remain (PL 95, fig. 12). 



The anterior or ventral plate of the cephalis (the " Hinterseite " of Biitschli), and 

 the posterior or dorsal side (the " Vorderseite " of that author), exhibit in the majority 

 of Spyroidea more or less marked difi'erences in the number, form, and disposition 

 of their pores, which require a far more accurate description than is here possible. 

 Usually the sagittal constriction of these two plates, produced by the primary vertical 

 ring, is deeper and sharper in the middle of the ventral than in that of the dorsal plate. 

 On both sides of the ring there appear in each plate usually two or three pairs of larger 

 pores, whilst numerous smaller pores are situated towards the lateral sides. We may 



