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Records of the Australian Museum (2009) Vol. 61 
Figure 18. A-J, Hydroides simplidentatus n.sp., from holotype AM W21415. (A) anterior end of 
worm; ( B-F ) different views of operculum; (G-7) bayonet-shaped special collar chaetae; (7), (right 
of figure) thoracic uncini, and anterior abdominal uncini (left of figure). 
Remarks. Hydroides simplidentatus n.sp. is somewhat 
similar to H. brachyacanthus Rioja, 1941. Sample BMNH 
1933.7.10.445 collected by C. Crossland from Coiba 
Island, Pacific coast of Panama, and determined as H. 
brachyacanthus Rioja, 1941, by H. Zibrowius in 1970, was 
compared with the present species. Although its corona is 
superficially somewhat similar to that of H. simplidentatus 
(Fig. 38K-P), in that a modified spine is enlarged swollen and 
curved towards the centre of the corona, close examination 
shows that they are quite different. In H. simplidentatus it 
is vesicular, like a parrot’s beak, hooked towards the centre 
of the crown, and enclosed in a translucent covering (Fig. 
18A-D,F). In H. brachyacanthus , on the other hand, the 
enlarged spine with its pointed tip is bent at right angles. In 
Hydroides cf. brachyacanthus Rioja from Grenada (Bastida- 
Zavala & Hove, 2002: fig. 29a-f; 2003: fig. 3a-d) 1 or 2 
enlarged coronal spines are directed ventrally. They are quite 
unlike the parrot beak-like modified coronal spine of H. 
simplidentatus, which is enclosed in a translucent covering. 
The structure of the remaining coronal spines in the two 
species is also very different. In H. simplidentatus they are 
in contrast, all the coronal spines in H. brachyacanthus 
are enlarged, although some may be simple, highly 
reduced, triangular, and having outwardly directed tips, 
conspicuously larger than the rest. They may also also have 
a knuckle-shaped bend and their blunt or pointed tips are 
directed towards the centre of the crown (Bastida-Zavala 
& Hove, 2002: fig. 29a-f; 2003: fig. 3a-d). According to 
the latter description (p. 154-155), “One dorsal spine is 
larger than the others, the rest gradually decrease in size 
(fig. 29b,d-f). Tip of spines pointed. Spines with one basal 
internal spinule (fig. 29f)_” Such basal processes are absent 
in the coronal spines of H. simplidentatus. Its modified 
coronal process lacks one, and there is no room left for them 
at the bases of the extremely reduced unmodified coronal 
processes which occur tightly pressed to the base of the 
modified coronal process. 
Straughan (1967a) reports S. brachyacanthus from eastern 
Australia, also incorporating material described by Dew 
(1959) under the same species. The latter also superficially 
resembles H. brachyacanthus and H. simplidentatus, but 
it is different from both of them. A collection by Dew 
from Townsville, Queensland, BMNH 1959.10.19.61-62, 
and determined as H. brachyacanthus, sensu Dew, by H. 
Zibrowius in 1970, was examined during this study. In one 
specimen, 4 of the more dorsal coronal spines are larger that 
the rest and curved over the centre of the crown. One of the 
latter, in turn, is only slightly larger than the rest. However, 
