74 
J.E. Watson 
median, length of base 
34-40 
median, depth of cup 
12-16 
median, diameter of cup 
30-34 
lateral, length of base 
38-42 
lateral, depth of cup 
10-16 
lateral, diameter of cup 
36-40 
hydrorhizal, length of base 
98-100 
hydrorhizal, depth of cup 
30-32 
hydrorhizal, diameter of cup 
28-36 
Remarks. The Mulder and Trebilcock collection in Museum 
Victoria contains five microslides (Canada Balsam-mounted) 
labelled ‘ Plumularia setaceoides var. crateriformis, 1910’. Two 
of these slides are labelled ‘type’ by Mulder and Trebilcock 
(see Mulder and Trebilcock 1910, p. 118). The locality of the 
specimens is Bream Creek on the central Victorian coast. 
Unfortunately, the specimens are so badly fragmented that it is 
impossible to determine whether they were originally 
Plumularia or Monotheca. These, and a third slide from the 
same series (NMV F57987, F57988, F57989), were designated 
syntypes by Stranks (1993). I select the best of these (F57987) 
as lectotype of Plumularia setaceoides var. crateriformis. 
Stechow (1925) raised the var. crateriformis to specific 
rank, recording the species from brown algae at 12-14 m 
depth in Warnbro Sound, Western Australia. Watson (2005) 
also reported P. crateriformis from algae at 16 m depth in the 
nearby Archipelago of the Recherche, Western Australia. 
Plumularia nodosa Stechow, 1924, also from Western 
Australia, is probably conspecific with P. crateriformis ; 
however as neither species is referrable to Monotheca as 
defined here, they are not considered any further. 
Two of the three remaining Mulder and Trebilcock 
microslides are labelled ‘ Plumularia setaceoides var. 
crateriformis, Bream Creek, January 1914’ and are presumably 
those on which Mulder and Trebilcock (1915) based their 
augmented description of the var. crateriformis. The third 
slide is labelled ‘ Plumularia setaceoides var crateriformis, 
Torquay, February 1915’. The Bream Creek specimens have 
only one hydrotheca on the hydrocladium and are referrable to 
Monotheca , whereas the Torquay specimen has more than one 
hydrocladial hydrotheca and thus belongs to Plumularia. The 
1914 Bream Creek specimens are identical to Monotheca 
amphibola from Queenscliff. 
The conspicuous anterior and posterior hydrothecal flanges 
of Monotheca amphibola resemble those in some of the smaller 
and more variable forms of the Plumularia setaceoides species 
group. These structures, together with the strong internal cauline 
segmentation and the wide, flat hydrorhiza with flexion joints are 
adaptations to strengthen the hydrocaulus in the high-energy 
Amphibolis seagrass habitat. The tall hydrorhizal nematothecae 
are probably for defence against the many grazing predators in 
the seagrass habitat. The smooth texture of the gonothecal wall 
is visible only in fresh material, as the gonothecae tend to 
crumple and become corrugated in mounting. 
Etymology. The species is named for the seagrass Amphibolis 
antarctica upon which it grows. 
Monotheca australis (Kirchenpauer, 1876) 
Fig. 1H-N 
Plumularia obliqua var. australis Kirchenpauer, 1876: 49, pi. 6, 
fig. 10.— Von Lendenfeld, 1885a: 474.— Stranks, 1993: 8. 
Plumularia australis Bale, 1884: 143, pi. 12, figs 7, 8.— Von 
Lendenfeld, 1885a: 475.— Bartlett, 1907: 42.— Mulder and Trebilcock, 
1916: 77, pi. 10, figs 1-lb.— Bedot, 1921: 26.— Stechow, 1921: 260.— 
Blackburn, 1938: 316.— Blackburn, 1942: 108.— Watson, 1973: 189.— 
Staples and Watson, 1987: 218.— Watson, 1992: 220. 
Plumularia compressa Bale, 1882: 31, pi. 15, fig. 5.— Bale, 1884: 
142, pi. 12, figs 9, 10, pi. 19, figs 39, 40.- Von Lendenfeld, 1885a: 
475.- Stranks, 1993: 9. 
Plumularia aurita Bale, 1888: 784, pi. 19, figs 16-19. 
Monothecella australis— Stechow, 1923a: 13.— Stechow, 1923b: 
221.- Leloup, 1932: 160. 
Monotheca australis.— Watson, 2003: 252. 
Material examined. The Kirchenpauer dry hydroid collection in 
Museum Victoria holds material (NMV F58239) labelled (presumably 
in Kirchenpauer’s handwriting) ‘ Monopyxis australis Port Phillip 
(Australien) on Zostera sp. c. 1865’. 
Material in author’s collection-, i) from the seagrasses Amphibolis 
antarctica and Amphibolis griffithv, Torquay, Victoria, depth 3 m, 7 
June 1970. Queenscliff, Victoria, depth 3 m, coll: J. Watson, 15 
October 1986. Portland, Victoria, depth 3 m, 14 May 1969. Tipara 
Reef, Gulf St Vincent, South Australia, depth 5 m, coll: J. Watson, 24 
November 1970. Starvation Bay, Western Australia, depth 2 m, coll: J. 
Watson, 11 January 1986. Point Peron, Western Australia, depth 3 m, 
coll: J. Watson, 26 January 1986. Whitfords Reef, Marmion, Western 
Australia, depth 4 m, coll: J. Watson, 6 February 1986. Cockburn 
Sound, Western Australia, depth 6 m, 12 October 1983. Starvation 
Bay, Western Australia, depth 6 m, coll: J. Watson, 13 January 1986. 
ii) from seagrasses Posidonia australis and Posidonia sinuosa-. Gulf 
St Vincent, South Australia, depth 10 m, coll: J. Watson, 14 December 
1968. Gulf St Vincent, South Australia, depth 12 m, 10 November 
1968. Whitfords Reef, Marmion, Western Australia, depth 3 m, coll: J. 
Watson, 22 November 1983. Whitfords Reef, Marmion, Western 
Australia, depth 24 m, coll: J. Watson, 30 January 1986. 
Description (from mounted and preserved material). Stems to 
7 mm high, arising from a hydrorhiza reptant on seagrass leaf, 
stolons broad and flat with flexion joints. Stems monosiphonic, 
unbranched, sometimes with a short athecate basal internode, 
following internodes all of same length, cylindrical, smooth, 
widening to distal apophysis; node above apophysis transverse, 
narrow. 
Hydrocladia alternate, in one plane, one distal on internode, 
given off below node on an upwardly directed apophysis with 
slightly oblique distal node. Hydrocladium with a short 
proximal athecate internode with thick walls and a strong, 
almost transverse distal node, hydrothecate internode 
considerably longer than athecate internode, base slightly 
convex, distal end blunt, ending below hydrothecal margin, no 
intranodal septa. 
Hydrotheca seated in a concavity of internode, occupying 
almost entire internode, slightly laterally compressed, 
posterior quarter of abcauline wall strongly convex, then 
becoming straight to margin, base of hydrotheca divided into 
two segments by a strong intrathecal septum passing obliquely 
backwards into hydrotheca from margin, a small circular 
foramen in floor of hydrotheca, location variable from mid- 
