66 



THE CYSTIDEA 



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

 the two on either side the anal interradius 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 



FIQ. XXXV. 



Cystoblastus Leuchtenbergi, after Volborth. 1, oral surface ; 2, posterior ; 3, aboral surface ; 

 4, analysis ; s.p, plates flooring ami-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 nmbones 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 exothecal, 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 



