TEE PLANKTON OF EASTERN NOVA SCOTIA WATERS 



7 



SESSIONAL PAPER No. 22a 



former genus some very large examples are met with^'; C. concinnus e.g. (Plate II., figs. 

 1 and 2) in which the sculpture of the valves is exceedingly fine. C. oculus iridis and 

 C. centralis are smaller and have more obvious sculpture which frequently suggests 

 artificial engine-turning (fig. 3). Actinoptychus undulatus Ralfs, Actinocyclus Ralfsii 

 Smith and Paralia sulcata (Ehrb.) Cleve are not uncommon (figs. 4, 21a and h, 23. 



The commonest species of Rhizosolenia was undoubtedly R. styliformis Bright- 

 well, in which the adjacent ends of the valves have very characteristic fitting surfaces 

 (Plate II., fig. 6), but R. setigera Bright, was also frequently represented, in which the 

 valves terminate in long spines with a peculiar spear-blade-like enlargement towards the 

 middle of their length (figs. 5 and 7). 



Still another type of plankton diatom is that which is furnished with delicate 

 bristles which enormously increase the amount of surface in contact with the water 

 without materially adding to the weight. To this type belongs the genus Chwtoceras, 

 which is not only rich in species but is profusely represented by individuals in the 

 plankton. 



ciKETOCERAS. — Ehven. 



In the following account of this essentially planktonic genus, I shall follow the 

 excellent paper of Gran (No. 4), which unfortunately I had not at my disposal when 

 I made the sketches of the forms observed at Canso. 



The genus is not only one of the most characteristic, but one of the most abundant 

 of plankton diatoms. It embraces a number of species, the synonymy of which is much 

 confused. I shall only attempt to enumerate those of the diagnosis of which I feel 

 certain. As Gran remarks, the arrangement of the chromatophores is often of con- 

 siderable diagnostic value : I have found this so in the sketches where it has been 

 noted. 



The various species of Chaetoceras generally form chains of more or fewer in- 

 dividuals. Each individual is a shorter or longer cylinder, more or less flattened, the 

 shell bounding which is formed of two valves with an intermediate hoop. The faces 

 of the valves where they come in contact with adjoining individuals are provided with 

 two bristles or setae, \^'hich interlock with the adjoining bristles and diverge from the 

 surlace of the chain at an angle generally characteristic for the species. The more 

 littoral species form spores which are peculiar in shape and decoration for the various 

 species, but no such spores were observed during the summer at Canso. 



Gran recognizes two subgenera Phaeoceras, in which the brownish chromato- 

 phores penetrate into the setae (which are frequently spinous), and Hyalochaete, in 

 which the setae are hyaline. 



To the former group belongs C. horeale Bail. (figs. 9 and 10), the cross section of 

 the cell of which is nearly cylindrical (24 x 22:5^), and the setae, which are over 

 5 mm. long and spinous, are situated in the sagittal plane. The foramina, gaps in 

 the chain between the individual cells, are hexagonal in outline. This form was com- 

 mon at Canso during July and August. 



Of the species belonging to the second group, I shall first refer to C. decipiens 

 (Plate II., fig. 8) which attracts attention on account of its considerable width which 

 I have measured up to 75//. The terminal bristles of the chain are shorter and 

 stouter, bear tnansverse striae, and are directed nearly parallel to the chain. It was 

 the commonest species observed at Canso. Less common members of the same group 

 were C. didymum Ehrb., C. laciniosum Schiitt and C. diadema Ehrb. The first 

 may be recognized by the lyrate foramina caused by a protuberance on the surface of 

 the concave valves as well as by the position of the two chromatophores which fit up 

 against these. In the second (fig. 1), the terminal setae are wider in the middle and 

 decorated with interrupted spiral lines of thickening. The third species betrays itself, 

 when seen from the valve-surface, by the circumstance that of the four setae two are 

 in a sagittal plane and two in opposite directions of the transverse axis. 



