1022 THE VOYAGE OF H.M.S. CHALLENGER. 



Family LII. ZYGOSPYRIDA, n. fam. (Pis. 84-87). 



Definition. S pyroidea without galea and thorax ; the shell consisting of the 

 bilocular cephalis only and its apophyses. 



The family Zygospyrida is by far the richest among the four families of Spyroidea, 

 the number of genera in the whole suborder amounting to forty-five, in the former to 

 twenty-eight ; and the number of species in the latter to two hundred and thirty-seven, 

 in the former to one hundred and seventy-two. The number of individuals also found 

 in many species of Zygospyrida is far greater than in any species of the three other 

 families. The shell of the Zygospyrida is represented by the bilocular cephalis only, and 

 never develops a galea (as in the Tholospyrida) nor a thorax (as in the Phormospyrida 

 and Androspyrida). The three latter families have therefore been derived from the 

 former as their common ancestral group. 



The Zygospyrida are very similar and nearly related to the Monocyrtida, and in both 

 groups the cephalis alone represents the whole shell. Therefore in 1882, Biitschli, in 

 the paper mentioned above (p. 1016), maintained the opinion, that these two groups were 

 identical. But there is this important difference between them, that in the Zygospyrida 

 (as in all S p y r o i d e a) the cephalis is bilocular, with a sagittal constriction, separating 

 the right and left chambers. In the Monocyrtida, however (as in all C y r t o i d e a), the 

 cephalis is unilocular, forming a quite simple chamber without sagittal constriction. Cor- 

 respondingly the primary sagittal ring in all Zygospyrida is well preserved and usually 

 complete, while in the Monocyrtida it is never complete, and often quite absent. 

 Another difference is indicated by the form of the central capsule, which in the 

 Zygospyrida is usually bilobed, and more developed in the frontal axis, whilst in the 

 Monocyrtida it is commonly ovate, and more developed in the principal axis. 



The important questions of the origin and phylogenetical relation of these two 

 similar groups of NASSELLARIA form a very complicated and difficult problem, and we 

 do not at present possess the means of solving it. It may be that a part of the 

 Monocyrtida has been derived from the Zygospyrida (as Biitschli erroneously supposes 

 for all), but the contrary is also possible. A third possibility is the independent origin 

 of both groups from the Semantida. But we shall see afterwards, in the description of 

 the Monocyrtida, that a great part of this group may with greater probability be derived 

 from the Plectoidea than from the Stephoidea, and that another part of them 

 has probably been derived quite independently from the Nassellida. Regarding the 

 complicated relations of these similar groups, mentioned above (pp. 892-894), it seems 

 hopeless here to discuss further their difficult affinities ; but in any case it seems useful 

 or even necessary to separate the Monocyrtida from the Zygospyrida, 



