On the Development of Dreissena polymorpha. 157 
two or three forms under it which must, I think, be accounted 
distinct *. Amongst the St.-Lawrence dredgings the true 
typical form is not by any means common; the species is 
represented chiefly by the variety portfera of Smitt. One 
specimen has occurred to me which, in addition to the ordi- 
nary characteristics of S. Landsborovit, is furnished with the 
large spatulate avicularia, which are rarely developed and 
always in connexion with the ocecium. 
Myriozoum planum, Dawson. 
Myriozoum crustaceum, Smitt, Krit. Forteckn., difversigt Kongl. Vet.- 
Akad. Forh. 1867, Bihang, p. 114. 
This species was first described as Lepralia plana by Sir 
J. W. Dawson in 1858. His account of the species is con- 
tained in a paper on the Polyzoa of the Gulf of St. Law- 
rence, published in the Report of the (Canadian) Geological 
Survey for 1858. It was afterwards (1860) reprinted along 
with other papers on the fauna of Canada by Messrs. D’Urban 
and Robert Bell, under the title ‘ Contributions to Canadian 
Natural History.’ This paper seems to have been entirely 
overlooked, and in 1867 Prof. Smitt described the present 
form under the name of Myriozoum crustaceum. Sir W. 
Dawson’s diagnosis may not be as full and minute as we 
should now desire, but it indicates the general character of 
the species, and his name has as good a claim to be retained 
as those of a large proportion of the older writers. 
EXPLANATION OF PLATE VIII. 
Fig. 1. Flustra solida, Stimpson, with sinus. la. Outline of orifice of 
smaller form, 16. Ditto of larger form. le. Showing the con- 
nexion between the epidermal covering of the cell and the 
tubular fibre. 
Fig. 2. Schizoporella cincta, Hincks, var. 
Fig. 3. Monoporella spinulifera, Hincks, var. preclara. 
Fig. 4. Membranipora armifera, Hincks. 
Fig. 5. Schizoporella cruenta, Norman. 
fig. 6. Porella concinna, Bisk, granular variety. 
Ig. 7. Lepralia pertusa, Ksper. 
XXVII.—On the Development of Dreissena polymorpha, 
Pallas. By Dr. KuGEN [$orScHELT f. 
Tue development of Dretssena is particularly interesting 
because, for reasons which will be discussed forthwith, we 
* Brit. Mar. Polyzoa, p. 545. 
+ Translated from a separate impression from the ‘ Sitzungs-Berichte 
der Gesellschaft naturforschender Freunde,’ no. 7, Jahre. 1891, pp. 131- 
146. Communicated by the Author, 
a 
