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



internal cavity is simple, without constrictions or septa. Among the Nassellaria they 

 occur in the Monocyrtida (Pis. 51-54, 98), where they haA^e received the name 

 " Cephalis." A form of shell, essentially the same, is found amongst the Ph^odaria in 

 the order P h se o g r o m i a, more especially in the Challengerida (PL 99), Medusettida 

 (Pis. 118-120), and Tusearorida (PI. 100), many of these latter closely resembling 

 many Monocyrtida. Such monothalamous Cyrtoid shells occur much more rarely among 

 the Spumellaria (e.g., among the Prunoideain Lithapium, Lithomespilus, Drup- 

 patractus,'2\ii. 13, 14, &c.). Polythalamous Cyrtoid shells (Pis. 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 Stichocj^rtid, 

 Zygocyrtid, and Polycyrtid. Zygocyrtid shells are characteristic of the Spyroidea 

 (Pis. 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, Pis. 64-75). 



124. The Circoid Skeleton. — This is a very important and remarkable type of 

 skeletal formation, which occurs exclusively in the legion Nassellaria, where it plays 

 a very prominent part ; its characteristic element is the " sagittal ring," a simple, 

 vertical, siliceous ring, which surrounds the central capsule in its sagittal plane, and is 

 specially differentiated in its basal portion. This "primary sagittal ring" whose vertical 

 aUopolar main axis coincides with that of the Monopylean central capsule embraced by 

 it, is characteristic of all members of the order Step ho idea (p. 931, Pis. 81-83, 

 92-94); here it forms by itself the skeleton of the Stephanida (PL 81); in the 

 Semantida (PL 92) it is combined with a horizontal basal ring, in the Coronida 

 (Pis. 82, 93) with a vertical frontal ring, and in the Tympanida (Pis. 83, 94) with 

 two horizontal rings, an upper mitral and a lower basal. In the great majority of these 

 Stephoidea there often develop in definite places characteristic processes or apophyses, 

 whose branches combine to form a loose tissue or an incomplete lattice-shell. This 

 becomes complete in the Cyrtellaria, the majority of which retain more or less 



