1130 THE VOYAGE OF H.M.S. CHALLENGER. 



The cephalis, or the first shell-joint of the C y r t o i d e a, is in the majority homo- 

 logons with the cephalis of the S p y r o i d e a, from which it differs iu the rednctiou of 

 the sagittal ring and the absence of the corresponding sagittal constriction ; its cavity 

 is therefore simple, not biloenlar. Its homology with the original cephalis of the 

 S^p y r i d e a cannot be doubted, when its base exhibits the typical basal pores of 

 the Semantida. But in many cases these are wanting, and in a great number of 

 C y r t o i d e a (mainly of Monocyrtida) there is more or less evidence that the original 

 cephalis is lost, and that the real first joint is the thorax, the original second joint. 

 At present it is quite impossible to distinguish between the former and the latter shells, 

 and therefore in the following descriptions the first joint is always named cephalis and 

 the second thorax. In future, when the affinities of the Cyrtoidea become better 

 known, it will be necessary to distinguish the " Archicej^halis," or the true cephalis of 

 all S p y r o i d e a and of the majority of Cyrtoidea, from the " Pseudocephalis " or 

 the false cephalis of the minority {e.g., of many Monocyrtida aperta, Archipilida, Archi- 

 phormida, Archicorida, &c.). 



The thorax, or the second shell -joint of the Cyrtoidea, is in the majority homo- 

 logous with the thorax of the Phormospyrida and Androspyrida, and therefore developed 

 by apophyses, which arise from the base of the cephalis and become united by transverse 

 branches forming a lattice-plate. Its size is generally in inverse proportion to that 

 of the cephalis. The more the cephalis becomes reduced, the more the thorax is 

 developed. Its form is very variable, usually three-sided pyramidal or prismatic iu 

 the triradiate, polyhedral in the multii-adiate, and conical or cylindrical in the eradiate 

 Cyrtoidea. Its terminal mouth is either a simple wide ojjening, or closed by a 

 lattice-plate. In the majority of C y r t o i d e a the thorax is separated from the cephalis 

 not only by the external collar constriction, but also by the internal cortinar septum, a 

 horizontal lattice-plate which exhibits the typical basal pores of the Semantida (usually 

 two smaller jugular and two larger cardinal pores). But this septum is often reduced 

 or perfectly lost, and then the external collar constriction alone indicates the se^iaration 

 of the cephalis and the thorax. 



The abdomen, or the third shell-joint of the Cyrtoidea, absent in the Mono- 

 cyrtida and Dicyrtida (as also in all S p y r o i d e a), occurs constantly in all Tricyrtida 

 and Stichocyrtida. It is a simple large chamber in the Tricyrtida, but forms an annu- 

 lated body, composed of a variable number of successive joints, in the Stichocyrtida. 

 The constrictions between these joints, and also the lumbar constriction, between abdomen 

 and thorax, are usually provided with a lattice-girdle, projecting into the cavity of the 

 shell, like a diaphragm. Usually this horizontal girdle bears only a single circle of pores, 

 rarely two or more. In many Cyrtoidea it is replaced by a solid horizontal ring of 

 silex, and often it is wanting. It originates by the insertion of the following shell-joint, 

 which takes place not on the terminal mouth of the preceding joint, but somewhat above it. 



