CARADOCIAN CYSTIDEA FROM GIRVAN. 383 



from precisely such a stage as the hypothesis postulates. Further, the trend of evolu- 

 tion in the median region is from a fairly elongate region of several irregularly alternat- 

 ing series of plates to a short region marked merely by a sharp transition from dimeres 

 to tetrameres. 



§ 87. Secondly, there are still occasionally to be found in D. Sedgwicki specimens, 

 such as E 16021 (PI. I. fig. 7), in which the ossicles composing the proximal region are 

 quite irregular in the neighbourhood of the theca, and seem to merge gradually into 

 the thecal elements. With these may be compared the curious stems from the 

 Cambrian of Ginetz, described by Barrande (1887, p. 181, pi. 2, f. 34) as Cigara 

 Dusli. The sharply tapering portion corresponds to the median region of the Dendro- 

 cystis stem ; the elongate, irregularly plated upper part, which Barrande -calls " une 

 partie du calice," corresponds to the proximal region of the stem, merging, no doubt, 

 into the theca. 



§ 88. I conclude, then, that in Dendrocystis we see preserved the successive stages 

 in the origin of at any rate the Heterostelean stem, A gradual elongation of the theca 

 produced a stem with a wide lumen enclosed by irregular ossicles. These gradually 

 assumed a regular alternation, and had Dendrocystis retained the upright habit, they 

 would probably have become arranged in alternating pentameres. But, in accordance 

 with the more prostrate habit, the ossicles were instead modified by successive stages 

 into dimeres, and the stem, togefher with the theca, assumed a bilateral character. 



Dendrocystis Barrandei n. sp. 



(PI. I. figs. 1-4. Text-figs. 6, 7.) 



1887. Dendrocystites Sedgwicki J . Barrande, " Echinodermes . . . Extraits du Syst. Silur.," 

 8vo, p. 223 (pars.). Also "Syst. Silur. . . . de la Boheme," vol. 7, "Echinodermes, 

 Cystidees," p. 145, pi. 27, K 1, 5, 19, 20, 21, 22, 23 (caet. excl.), and ? pi. 38, f. 16. 



§ 89. Diagnosis. — A Dendrocystis with thecal width about '9 of thecal height ; thecal 

 lobes descending slightly below stem-attachment; antibrachial shoulder a rounded 

 hump ; thecal plates all smooth, with greatest diameters from 1 to 4 mm., but most of 

 them approximately equal in size. Length of brachiole rather less than thecal height ; 

 width tapering from ca. 2-3 mm. to '4 mm. and less. Length of stem about twice 

 thecal height ; distal region subcylindrical, with dimeres subalternate ; median region 

 expanding gradually, with dimeres regularly alternating or breaking up irregularly ; 

 proximal region annulate, with smaller plates between the rings. 



§ 90. Locality.— Trubsko and ? Hajek (Barrande, pi. 38, f. 16), Bohemia. 



§ 91. Horizon.— d 2, quartzites, corresponding in time about to the beginning of the 

 Llandeilian. 



§ 92. Material— Holotype, an imprint of theca with appendage and part of stem 

 from Trubsko, Brit. Mus. E 16026 (PI. L fig. 1). 



TRANS. ROY. SOC. EDIN., VOL. XLIX. PART II. (NO. 6). 51 



