On the Development o/'Dreisscna polymovplia. ir>7 



two ov tlivce forms under it wlilcli must, I think, be accounted 

 distinct ^. Amongst the St. -Lawrence dredgings the true 

 ty|)ical form is not by any means common ; the species is 

 represented chiefly by the variety porifera of Smitt. One 

 specimen lias occurred to me whicli, in addition to the ordi- 

 nary characteristics of S. L<indshorovii^ is furnished with tlie 

 large spatulate avieularia, which are rarely developed and 

 always in connexion with the ooecium. 



Myriozoum planum^ Dawson. 



Myrinzomn rrustarctim, Smitt, Krit. Forteckn., Qifversigt Kongl. Vet.- 

 Alcad. Foih. 1867, Biliaug, p. 114. 



This species was first described as Lepralia plana l)y 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 18oS. It was afterwards (I860) reprinted ahong 

 with other papers on the fauna of Canada by Messrs. D'Urbaii 

 and Robert Bell, under the title ' Contributions to Canadian 

 Natural History.' This paper seems to have been entirely 

 overlooked, and in 18G7 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. Fluatra solida, Stimpson, with sinus, la. Outline of orifice of 

 smaller form. lb. Ditto of larger form. Ic. Showing the con- 

 nexion between the epidermal covering of the cell and the 

 tubular fibi'e. 



Fiff. 2. Schizoporella cincta, Hincks, var. 



FHy. 3. MonoporeUa spimdifera, Hincks, var. pradara. 



Fig. 4. Memhranipora armifera, Hincks. 



F\g. 5. Schizoporella crucnta, Norman. 



Fig. 6. Porella concinna, B tsk, granular variety. 



Fig. 7. Lepralia pertusa, Esper. 



XXVII. — On the Development o/'Dreissena polymorpha, 

 Pallas. By Dr. Eugen Kokschelt t- 



The development of Dreissena is particularly interesting 

 because, for reasons which will be discussed forthwith, we 



* Brit. Mar. Polyzoa, p. 345, 



t Translated from a separate impression from the ' Sitzungs-Berichte 

 der Gesellschaft naturforschender Freunde,' no. 7, Jahrg. 1891, pp. 131- 

 146. Communicated bv tlie Author. 



