SOME XORTH AMERICAN DESMIDIE^. 235 



•20. Pleurot^nium subcoronulatum, West & G. S. West, in Tmns. Linn. Soc. scr. 11. 

 (Bot.) V. (1895) p. U, pi. 5. fig. 33. {Docidium subcoronulatum, W. J^. Tuni. 

 Ereshw. Alg. of E. India, p. 29, t. 3. fig. 1.) 



Var. DETUM, nov. rar. (PI. XIII. figs. 2, 3.) Var. semicellulis subito constrictis infra 

 apices dilatatis, constrictione ad partem septimam longitudinis (circitcr) a polo 

 sit^ ; tuberculis 26-30 (visis — 14-16). 



Long. 404-446^ ; lat. ad bas. semicell. 29-38-5 ;« ; lat. ad constrict, infra apic. 20-27 ^i ; 

 lat. ad apic. 27-28 /u. 



21. P. PARALLELUxM, West & G. S. West, in Trans. Linn. Soc. scr. 11. (Bot.) v. (1895) 

 p. 45, pi. 5. fig. 34. 



Long. 727-891 u ; lat. ad bas. semicell. 50-52 ^ ; lat. ad apic. 36-39^. 



22. P. ScEPmuM, nobis. (Docidium Sceptrum, Kuetz. Species Algarum, p. 168 ; Hoy, 

 in Scottish Nat. (1883-4) p. 37 ; Docidium tridentulum, Wolle, Desm. U.S. p. 52, 

 pi. 10. fig. 10 ; Fleurotcenium tridentiUum, West, in Journ. Linn. Soc. (Bot.) xxix. 

 (1892) p. 120.) 



A figure of a typical specimen is given. 



Long, (sine dent.) 250 ju; lat. ad bas. semicell. 12-5 |u; lat. apic. 7"5^. (PI. XIII. fig. 0.) 



Var. CAPITATTJM, West. {Pletirotcenium tridentulum var. capitatuMj West, I. c. pi. 24. 



fig. 12.) (PL XIII. figs. 7, 8.) 

 Long. 330-360 |Ci; lat. ad bas. semicell. 12-135 /it ; lat. ad apic. 7'5-8'5m- 



'23. P. TROCHISCTJM, nov. sp. (PL XIII. figs. 4, 5.) P. mediocre, circiter diametro 15*16- 

 plo longius; semicellulse cylindricae lateribus rectis; vix attenuatum polis, cum 

 inflatione parva ad basin, apicibus truncatis; raembrana annulis 13-15 imprcssionum 

 irregularinm subquadrangularium vel oblonganira (non in scries longitudinaliter 

 ordinatis), circiter 6 annulo unoquoqne (impressiones partes tenuissimcc membranoe 

 sunt — verrucas non sunt), impressionibus intra inflationcm basalem minoribus 

 irregularibus et numerosioribus, lis intra polos irregulariter elongatis (interdum 

 fractis) in parte octava semicellulae uniuscujusque. 

 Long. 416-458 n ; lat. ad bas. semicell. 29-38 ^ ; lat. med. semicell. 25-31 ft ; lat. apic. 

 21-27 iu. 



The somewhat quadrangular markings on this species are due to those portions of the 

 membrane which are not internally thickened, and, being the thinnest parts of the 

 membrane, are analogous to the pits in the large vessels of the xylem of angiosperms. It 

 diff'ers from Fleurotcenium tessellatum, Lagerh. (i. e. Docidium tessellatnm, Josli., in 

 Journ. Linn. Soc. (Bot.) xxi. (1886) p. 650, t. 25. fig. 15), in having a distinct basal 

 inflation, in not being attenuated to the apex, which is, moreover, without teeth. The 

 rings of markings are also more numerous : the markings are fewer in each ring, they are 

 not regularly oblong and are not in longitudinal scries. Lagerheim (in Ofvers. Sv. Vet.- 

 Akad. Eorh. (1887) n. 8, p. 541) states that the elongated warts at the apex of 

 P. tessellatum are only faintly indicated ; in our plant they are most clearly indicated 

 and only occupy about an eighth part of the semicell. It diff'ers from P. cerrucosum 



