6 Proceedings of the Royal Physical Society. 
I1.—On the Invalidity of the Hydroid Genus, Diplopteron, Allman. 
By James Ritchie, M.A., D.Sc., Royal Scottish Museum. 
(Read 25th November 1912. MS. received 25th November 1912.) 
THE presence of more than one pair of lateral sarcothecee has been observed - 
in several Plumularian species. On this account, together with the possession 
of doubly pinnate ramification, Allman created the genus Diplopteron} 
Bale suggested, “if this genus be retained . . . that it be modified so as to 
comprise all species with more than one pair of lateral nematophores” ; ? 
and Nutting accepts this character as distinctive of the trophosome, adding, 
as characterising the gonosome, “gonangia protected by accessory ramuli 
borne on the hydrocladia, and bearing a few scattered hydrothece.”* He 
specifically excludes Plumularia sulcata, Lamk. (=aglaophenoides, Bale) from 
this genus, and would place it in a genus by itself on account of its fixed 
mesial sarcotheca. 
But what is the true status of the “two or more pairs” of lateral sar- 
cothecze amongst the Plumularide ? They occur after two main types. The 
primary pair in each type is similar: elongate sarcothece lying one along 
each side of the hydrotheca, and supported upon a process from the 
hydroclade adnate to the hydrotheca wall. But the additional pair or pairs 
may be indefinite or definite in position. In the former case, the additional 
sarcothecz rise distinctly from the internode wall some distance above the 
primary sarcothece, their position as a pair is not fixed in any species, they 
do not lie necessarily in the same relative position to each other, eg. the 
individuals of a pair frequently do not originate in the same horizontal plane 
at right angles to the long axis of the internode, and in some species, at least, 
the pair may be replaced by a single median individual. In such a group 
fall Plumularia suleata, Lamarck (=aglaophenoides, Bale), Polyplumularia 
flabellata, G. O. Sars, Diplopteron grande, Nutting, Diplopteron quadricorne, 
Nutting, and Diplopteron longipinna, Nutting. 
In the second type, the additional pair of sarcothecze rises just at the base 
of the processes supporting the primary lateral sarcothece. Since these 
processes are fixed, the positions of ‘the sarcothecze as individuals and as a 
pair are thus definite and fixed, and in the same horizontal plane. Nor has 
such a pair been observed to have been replaced by a single median 
individual. The additional sarcothecs here are distinctly lateral, and may 
1 Allman, Trans, Zool. Soc., London, 1874, vol. vill. p. 479. 
2 Bale, Cat. Australian Hydroid Zoophytes, Sydney, 1884, p. 124. 
3 Nutting, American Hydroids, pt. i., Washington, 1900, p. 81. 
