REPORT ON THE RADIOLARIA. 1131 



The annular joints of the Stichocyrtida succeeding the third joint, and very 

 variable in number, may be regarded either as a series of new postabdominal chambers, 

 succeeding the true abdomen, or as secondary joints of the annulated abdomen itself. 

 The latter view may be sustained by the fact that these joints are usually of an indif- 

 ferent shape, and do not possess the characteristic features which we find in the first three 

 joints, the abdomen, the thorax and the cephalis. 



The lattice-work of the shell exhibits in the Cyrtoidea an extraordinary variety, 

 similar to that of the Sphseroidea; it serves in the first place for the distinction of 

 species. The three first joints of the shell are often distinguished by the different char- 

 acter of the lattice-work. The cephalis has usually very small and simple pores. The 

 lattice-work of the thorax is often characterised by radial structures. The pores of the 

 abdomen are usually very numerous and regular. The numerous joints in the annulated 

 abdomen of the Stichocyrtida commonly exhibit little vai'iety. 



The closure of the mouth, effected by a convex or horizontal terminal lattice-plate, 

 has a different signification in the Monocyrtida and in the jointed Cyrtoidea. In the 

 Monocyrtida clausa this closing plate is the original cortinar plate or the basal plate of 

 the cephalis. In the jointed Cyrtoidea, however, the lattice-plate which closes the 

 terminal mouth of the thorax or of the abdomen (of the last annular joint in the Sticho- 

 cyrtida), is produced by central union of the convergent edges, which grow centripetally 

 from the margin of the mouth of the last joint towards its centre. 



The radial apophyses arising from the shell of the Cyrtoidea may probably 

 be always derived from that tripodal structure which is found in all P 1 e c t o i d e a, in 

 Cortina and Cortiniscus among the Stephoidea, and in the majority of S p y r o i d e a, 

 Therefore the prototype of this radial structure would be Plagoniscus and Cortina, with 

 four radial spines united in a common point, the cortinar centrum ; an ascending apical 

 horn and three descending basal feet. The odd posterior or caudal foot is usually 

 similar in shape to the two paired anterior or pectoral feet, but may be distinguished 

 from these latter by its relation to the apical horn. Very frequently an internal vertical 

 free columella arises in the cephalis, or instead of it an ascending rib in the dorsal wall 

 of the cephalis, which connects the base of the apical horn with the origin of the caudal 

 foot. This is probably the remaining part of the sagittal ring. More rarely also a part 

 of the ventral rod of the latter is preserved, or on the anterior pole of the basal rod of 

 the cephalis an ascending procolumella arises which is inserted on the frontal face of the 

 cephalis, and sometimes prolonged into a nasal horn (the rod, C, of Biitschli). These 

 two odd horns, the posterior apical horn and the anterior nasal horn, are usually different 

 and divergent. In some genera a variable number of accessory radial horns is developed 

 on the convex face of the cephalis. In many hornless genera the free apical horn is 

 lost, but not unfrequently the columella is preserved which connects the caudal foot 

 with that point of the cephalis, in which formerly the apical horn was inserted. 



