123. _The Cyrtoid Skeleton._--Cyrtoid skeletons are those lattice-shells which possess a vertical main axis with two different poles (Monaxonia allopola); the upper pole is usually termed the apical, the lower the basal. Such Cyrtoid shells are characteristic of the great majority of the NASSELLARIA or MONOPYLEA (and especially of the #Cyrtellaria#); they are also found in a large division of the PHÆODARIA (the #Phæogromia#), and in some SPUMELLARIA. In general the manifold Cyrtoid shells may be divided into two large groups, those with one and those with several chambers. The _monothalamous_ Cyrtoid shells are usually ovoid, conical, cap- or helmet-shaped; their {lxxxii}internal cavity is simple, without constrictions or septa. Among the NASSELLARIA they occur in the Monocyrtida (Pls. 51-54, 98), where they have received the name "Cephalis." A form of shell, essentially the same, is found amongst the PHÆODARIA in the order #Phæogromia#, more especially in the Challengerida (Pl. 99), Medusettida (Pls. 118-120), and Tuscarorida (Pl. 100), many of these latter closely resembling many Monocyrtida. Such monothalamous Cyrtoid shells occur much more rarely among the SPUMELLARIA (_e.g._, among the #Prunoidea# in _Lithapium_, _Lithomespilus_, _Druppatractus_, Pls. 13, 14, &c.). Polythalamous Cyrtoid shells (Pls. 55-80) occur exclusively in the NASSELLARIA, and exhibit in this legion an astonishing variety of structure; they are distinguished from the monothalamous forms by the development of internal septa, or of annular incomplete diaphragms, which usually correspond to the external constrictions; their interior is thus divided into two or more communicating compartments. Among the polythalamous Cyrtoid shells may be distinguished three principal groups, the Stichocyrtid, Zygocyrtid, and Polycyrtid. Zygocyrtid shells are characteristic of the #Spyroidea# (Pls. 84-90), and are distinguished by a bilobate cephalis (cephalis bilocularis); the median sagittal ring, or a corresponding constriction, divides the shell into right and left compartments. Polycyrtid shells (Pl. 96) are peculiar to the #Botryodea#, and characterised by a multilobate cephalis (cephalis multilocularis). Stichocyrtid shells are those in which the primary cephalis remains simple, and new joints are successively added to its basal pole; such shells occur in the majority of the #Cyrtoidea#. Secondary chambers are sometimes added in the other two groups (#Botryodea# and #Spyroidea#). When, as often happens in these polythalamous Cyrtoid shells, two or three distinct joints follow each other, the first is called the "cephalis," the second the "thorax," and the third the "abdomen" (Tricyrtida Pls. 64-75).