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



and essential part of the skeleton, but secondary and of little morphological value, 

 absent in the majority of the Semantida and of the other NASSELLARIA. 



We divide our family Semantida into two different subfamilies, which possibly 

 possess a direct phylogenetic relation to the two subfamilies of Stephauida : The 

 Semantiscida have no typical feet, and have arisen directly from the Lithocircida ; the 

 Cortiniscida, however, possess the three typical basal feet of Cortina, and may there- 

 fore be derived directly from the Cortinida. Since these three cortinar feet are 

 probably identical with the three primary radial rods of the Plectoidea, an imme- 

 diate affinity also to these NASSELLARIA is indicated. 



The Semantiscida, which do not possess these three basal cortinar feet, are the simpler 

 forms of the family. The simplest of all, and perhaps the common ancestral form of 

 the whole family, is Semantis (PI. 92, figs. 1, 2). It may be derived from Archicircus 

 or Zygocircus by development of two pairs of horizontal apophyses on its base, around 

 the porochora of the central capsule. The two rods of each side (right and left), 

 becoming curved one towards the other, and meeting laterally, form a simple horizontal 

 gate, and the two paired basal gates together, a horizontal ring or basal ring, to which 

 the primary sagittal ring is perpendicular. In the next allied genus, Semantrum (PL 

 92, figs. 35), three pairs of horizontal apophyses are developed, and therefore two 

 pairs of basal gates produced, an anterior and a posterior. In the third genus, 

 Semantidium (PL 92, figs. 6, 7), three pairs of basal pores or gates are visible, 

 surrounded and separated by four pairs of horizontal apophyses, which arise from the 

 base of the sagittal ring. Finally, in Clathrocircus (PL 92, figs. 810) the number 

 of apophyses is much increased, and two parallel rows of pores are developed along the 

 two sides of the sagittal ring. 



The basal plate or the "seal," developed from the base of the primary sagittal ring 

 or " signet-ring," is therefore a horizontal ring, which becomes bisected by the latter, 

 and exhibits either one pair of primary " basal gates " or two or three pairs of 

 these important basal pores, rarely more. Since these pores possess the greatest 

 morphological value, and are probably everywhere homologous, we give to them and to 

 the separating apophyses certain names, and call the anterior pair of gates, "jugular 

 pores " (i in our figures, the pair I of Biitschli) ; the middle (usually the largest) pair, 

 " cardinal pores " {& in our figures, the pair II of Biitschli), and the posterior, smaller 

 pair, " cervical pores," I. The typical pairs of rods, by the union of which these basal 

 pores arise, are the following : (1) the clavicular or furcular rods,/, the first pair (rods 

 el of Biitschli), (2) the coracal rods, e, between the jugular and cardinal pores (rods 

 e of Biitschli), (3) the scapular rods, g, between the cardinal and cervical pores (rods 

 e2 of Biitschli), (4) the cervical rods, the fourth pair of apophyses, the most posterior, h. 

 Biitschli supposes that the topographical succession of the three typical pairs of 

 basal pores is also the chronological succession, the jugular being formed first, the 



