66 



THE CYSTIDEA 



with dicyclic base (p. 99), and dominated by trimerous symmetry. IBB 4, 

 the two on either side the anal inter-radius apparently being produced by 

 fusion of two original pairs. BB 6 (ten in Heterocystis). Alternating with 

 BB are 6 RR, between which, on the anterior side of the cup, two or three 

 interradials (iR) may be developed. (In Heterocystis the iRR also alter- 

 nate with the BB.) On the interior of all these plates the stereom is 

 thrown into strong folds, forming bundles of laminae at right angles to 

 the sutures ; on the exterior the ends of the folds are marked by pores, 

 each surrounded as a rule by a raised rim, and sometimes broken up into 

 two or more smaller pores. Since the laminae correspond in position and 



Fio. XXXV. 



Cyitoblastus Leuchtenbergi, after Volborth. 1, oral surface ; 2, posterior ; 8, aboral surface ; 

 4, analysis ; s.p, plates flooring arm-grooves ; 0, mouth ; As, anus, the position of which is 

 indicated by * in 1 ; M, hydropore between 18 & 19 ; St', attachment of stem. 



essential structure to ordinary pore-rhombs, the pores necessarily run in 

 lines from the umbones to the angles of each plate (Fig. XXXVI. 3, 4) ; these 

 structures may have helped in respiration ; but are in no sense homo- 

 logous with Blastoid hydrospires. The three primitive food-grooves (ant., 

 r. and 1.) are of equal size and bifurcate in like manner. The proximal 

 portion tends to be hypothecal ; then a short portion lies on the theca 

 like the " recumbent arms " of the Glyptocystidae ; the distal portion is 

 freely erothecal, biserial, and brachioliferous. Stem well developed, 

 circular in section. Genera Hemicosmites, von Buch (1840, redescribed 

 1845 ; see also Joh. Miiller, 1854 ; syn. Hexalacystis, Haeckel), Lower to 

 Upper Ordovician, Russia, and perhaps Silurian, N. America. Com- 

 paring the dorsal cup (Fig. XXXVI. 3) with a crinoid cup, we may 



