Naviculacea.'] infusorial animalcules. 311 



Diameter of fossil, as low as l-8G0th; of living, as high as 1 -580th. 

 This species differs from A. duodenarius by the absence of septa. 



AcTiNOCTCLUS duodenariiis. — Disc divided internally, by partitions, 

 into twelve cells, and having twelve finely punctated rays; six dark and 

 six bright triangidar divisions are seen, in the centre of each of which 

 rnns a narrow line, terminating at tlie margin in a little opening. 

 The internal partitions appear to lie between every two of these 

 narrow lines, so that as many as twenty-four rays may be counted, 

 but there are only twelve openings visible. Diameter 1 -560th to 

 1 -480th. 



A. quindenarms. — Partitions not present ; rays fifteen, finely 

 punctated. Fossil and alive in the localities named. The discs 

 of this species are more arched than those of any other of the pre- 

 ceding ones. The fifteen rays terminate in fifteen marginal open- 

 ings. The granules are distributed into forty-eight round, yellowish, 

 brown-coloured masses, placed around the bright central spot of the 

 body, or else appear united as one ball. Locomotion not perceptible. 

 Diameter of fossil foniLS, 1 -560th; of living, 1 -560th to 1 -480th. 



A. sedenarius. — Divided by internal partitions into sixteen cells, 

 and having sixteen finely punctated rays. The granules of a green 

 colour, form, in some, separate concentric masses ; in others, a single 

 ball-like mass, placed in the middle of the body. Locomotion not 

 perceptible. Alive near Cuxhaven. Diameter 1 -290th. 



A. ododenarius. — Divided by internal partitions into eighteen cells, 

 and having eighteen finely punctated rays. This species is very 

 similar to the preceding, being only a little larger, and having 

 eighteen septa and marginal openings. The granules in one specimen 

 consisted of seven large yellowish green masses, placed concentrically 

 around the bright central spot of the body, but which did not appear 

 to be strictly confined to the divisions or cells, which, most jirobably, 

 arises fi'om some optical deception. It was remarkable that, in this 

 specimen, the openings were situated at the margin, in the centre of 

 each division. The play of colour of these divisions depends upon 

 some optical phenomena yet to be developed. In the centre of the 

 disc of those species provided with these divisions, is a broad, bright, 

 and polished umbilicus-like spot, which is invisible in those not pos- 



