In the periphery of the voluminous calymma of this singular genus a large number of globular cells are developed (figs. 7, 10), probably symbiotic xanthellæ. In some forms of _Nephrospyris_ these xanthellæ are enclosed by a peculiar inflated girdle, developed in the periphery of the kidney-shaped skeleton (Pl. 90, figs. 1, 4-6). _Synopsis of the Families of_ #Spyroidea#. {Cephalis without galea Shell without thorax; no second { (or apical cupola), 1. ZYGOSPYRIDA. shell-joint, developed from { the base of the cephalis. {Cephalis with a galea { (or apical cupola), 2. THOLOSPYRIDA. {Cephalis without galea Shell with a thorax, or a { (or apical cupola), 3. PHORMOSPYRIDA. second shell-joint, developed { from the base of the cephalis.{Cephalis with a galea { (or apical cupola), 4. ANDROSPYRIDA. {1022}Family LII. #ZYGOSPYRIDA#, n. fam. (Pls. 84-87). _Definition._--#Spyroidea# 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, Bütschli, 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 #Spyroidea#) the cephalis is bilocular, with a sagittal constriction, separating the right and left chambers. In the Monocyrtida, however (as in all #Cyrtoidea#), the cephalis is unilocular, forming a quite simple chamber without sagittal constriction. Correspondingly 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.