CARADOCIAN CYSTIDEA FROM GIRVAN. 377 



structure far more legitimately to be homologised with one of the corner plates of 

 Ceratocystis. In D. scotica, for instance, in G 112 the lobes have a very rigid appear- 

 ance, and one of them ends in almost a sharp point ; these are composed of more than 

 one plate, but in form and relation to the stem they correspond precisely to the lower 

 horns of Cei'atocystis. Again in D. Sedgivicki and D. Barrandei some specimens show, 

 whether on the brachial or antibrachial side, at the lower angle of the theca, a rather 

 definite appearance, as though the lobe were here strengthened by somewhat thicker 

 plates. (See further §§ 77-79, and PI. I. figs. 2, 5, 9, and text-figs. 6, 8.) 



§ 59. (2) Basals. Dr Jaekel says of his Carpoidea (1901, p. 662), "Die Basis ist 

 vier- oder dreitheilig"; and more particularly of his Heterostelea (p. 666), " anscheinend 

 immer 4 Basalien." The Dendrocystidae, however, fall within his Suborder Soluta, of 

 which he says (p. 672), "Die Ordnung des Thecalskeletes vollstandig aufgelost." The 

 facts appear to be these : — There are no plates so distinct in number and shape as to 

 be called "basals." The proximal plates of the theca do not, however, merge into the 

 stem (unless in exceptional cases) but are distinguished from it by a thickened proximal 

 margin. This often forms a slight rim on the outer surface of the theca (PI. 1. figs. 1, 

 3, 4 ; PI. n. figs. 19, 24) ; and some specimens suggest that there was also a projection 

 into the lumen, though this may have been confined to one or more of the larger plates. 

 For it is the case that at least one (and it may be two) of these plates is frequently 

 larger than the others (PI. I. figs. 1,5; PL II. fig. 12). This seems to be a permanent 

 feature in the earlier species, D. Bar7-andei, where the adjoining plates are small and 

 numerous, so that the enlarged plate is conspicuous by its size and surroundings as 

 well as by its strongly projecting rim. There is a similar plate (or it may be plates) 

 in many, perhaps all, individuals of D. Sedgwicki, but the rim is not so disproportion- 

 ately prominent. In these two species there may be fully ten plates abutting on the 

 stem. In D. scotica the number is probably not more than six as a rule, and though 

 some are often larger than the other thecal plates, there does not appear to be one 

 permanently distinguished by its size or prominence. 



§ 60. The basal rim is presumably a stiff"ening against mechanical pressure. Partly 

 it may be intended to prevent collapse of the lumen, and consequent crushing of the 

 chambered organ, which we may suppose to have lain, as usual, at this level. The 

 internal projection, however, restricted as it seems to be to a limited tract, suggests a 

 point of attachment for muscles or ligaments connecting the stem with the theca. 



§ 61. (3) Adbrachial plates. At the base of the appendage the thecal plates are 

 thickened ; all or some are frequently larger, and may even, in D. Sedgwicki, have 

 been folded (PI. I. figs. 4, 6 ; PL II. figs. 11, 18). They must have served either as a 

 platform to support the appendage steadily and as a fulcrum for muscular action, or as 

 a frame to prevent the lumen from being crushed — possibly in both capacities. 



§ 62. As in the case of the plates adjoining the stem, the larger among these plates 

 often, and especially in D. Barrandei, abut on much smaller, somewhat elongate plates 

 (PL I. fig. 1). In each of these cases rigidity and support are obtained just where they 



