CAKADOCIAN CYSTIDEA FROM GIRVAN. 



389 



§ 115. The plates have a smooth surface, and most of them are flat or equably- 

 curved as a rule, but may be slightly swollen. The umbonate and folded plates in 

 the lower half of the theca are seen in the holotype, in EI6OI8, E 1(5020, E 16031, and 

 in Barrande, pi. 26, ff". 1, 8, 18; pi. 27: ff. 3, 6, 12. The only evidence available 

 (E16031, and Barrande, pi. 26, fi". 1, 6, 18; pi. 27, f. 6) suggests that when these 

 face the observer, the brachial appendage is on the left (PI. I. figs. 5, 6). The apparent 

 absence of umbonate plates from some specimens suggests that they occur on only one 

 face of the theca. 



The Stereom has already been described (§§ 53-56). 



§ 116. The statements as to the basal plates of the theca (§ 59) rest on the following 

 evidence. The exposed face of the holotype shows four or five plates adjoining the 

 stem (PI. I. fig. 5), and this is confirmed in a general way by the British Museum 

 specimens, though they are obscure. A large plate, with smaller ones appearing to 

 radiate from it, is seen in the holotype and in E 16019 and E16020; compare also 

 Barrande, pi. 26, ff". 2, 4, 10, and pi. 27, ft". 3, 6. 



§ 117. Nothing can be added to the account of the ad brachial plates (§ 61). 

 They are well shown in E 16021 (PI. I. fig. 7), and in one of the individuals on 

 E16019. 



§ 118. The Brachial Appendage is well shown in the same individual of E16019, 

 and the facts are given in §§ 63-66. In the holotype the proximal elements are tele- 

 scoped in, so as to imbricate distalwards, but this is probably a mere accident of fossil- 

 isation. The apparent ridging on the interior of the ossicles is seen in an isolated 

 appendage on El 6018 (PI. I. fig. 9). Slight exterior ridging appears to be shown in 

 Barrande, pi. 26, ff. 15, 22. 



§ 119. Measurements of the Brachial Appendage in millimetres are : — 





Length. 



Proximal Width. 



Distal Width. 





E 16018 



E 16019 



Bahrandb, pi. 26, f. 16 



14 + 

 17 + 

 26-6 



3-5 

 5 



4.4 



2 



1-9 

 •9 



incomplete 

 incomplete 

 measured from 

 the figure 



The proximal width is much greater in some of Barrande's figures ; as much 

 as 8 mm. in pi. 26, f. 2. The appendage was, no doubt, of compressed elliptical 

 section. 



§ 120. The Vent has been fully discussed (§ 74). There are suggestions of the 

 sugar-loaf plate in two individuals on E 16019 ; but, except for the elongate plates in 

 E 16031 (PL I. fig. 6), the specimens accessible to me do not shed much light on this 

 obscure problem. 



Here reference may be made to the possible structures connected with the antanal 

 lobe of the theca (§§ 76-79). 



