4 8 



THE CYSTIDEA 



importance : The theca suddenly thins below to about one-third its 

 width, forming a tubular extension (St), the walls of which contain 

 numerous small plates, which gradually become 

 larger and more definite in arrangement, and 

 merge into a tube with a narrow lumen en- 

 Q T ttg^ closed by comparatively large solid plates. Com- 



parison with more highly developed genera 

 shows that this extension is a stem (columiwC), 

 and its presence indicates a more fixed habit 

 than was, perhaps, assumed by Aristocystidae. 

 Further influence of fixation is clear in the de- 

 velopment at the opposite pole of the theca 

 of a movable, jointed tube (Br\ composed of 

 four or five rows of small plates, wider than 

 high, and often alternating : this tube tapers to 

 a rounded end, in which no opening is per- 

 ceptible ; neither are there openings between 

 its plates ; the plates may, however, have 

 opened along one of the vertical lines, thus 

 converting the tube into a groove, exactly as 

 figured in Fig. 5 of Barrande's Plate XXVI. (see 

 Fig. IX.). This organ was regarded by Barrande 

 and Trautschold as a tubus ventralis for genital 

 (not faecal) products ; by Neumayr (1889) as 

 an arm, with a double row of ambulacral pores ; 

 by Haeckel (1896) as an oral proboscis, or 

 possibly the stem. It is here regarded as an 

 extension from the mouth, bearing a ciliated 

 food-groove that could be closed by plates, and 

 perhaps also an extension from the water-ring. 

 Other thecal openings are doubtful ; an anal 

 pyramid may have existed in the lower third 

 of the theca (As), but Barrande's figures and 

 descriptions are inconsistent ; hydropore- and 

 gonopore quite unknown. Folds or ridges 

 radiate from the centre to the edges of each 

 thecal plate besides strengthening the plates, 

 these folds, like similar ones in later stalked 

 forms, may indicate the concentration of a 

 nervous layer in the integument into definite 



-Br 



FIG. IX. 



stored 



Barrande. 



pe 



Dendrocystis Sedgwicki, re- , / i a- J.-L ^-L i 



red on the evidence of tracts (axial nerves) putting the stem, thecal 



rrande. Br the arm-like ap- p i a tes, and plates of food - grooves into con- 

 ndage, copied from Barrande. * _ * 



nection. Ghgara, Barrande (1887), resembles 



the stem of Dendrocystis, and suggests its occurrence in the Cambrian. 

 Syringocrinus paradoxus, E. Billings (1859), is the same thing from Quebec. 

 FAMILY 3. EOCYSTIDAE. Established to include certain obscure forms 

 from the Lower and Middle Cambrian of Great Britain and North America. 

 Eocystis, Billings (1868), and Protocystis, Hicks (1872, see also Salter, 1873), 

 have never been properly described or figured ; but since they cannot 



