52 



DEPARTMENT OF THE NAVAL SERYICE 



TABLE IV. 

 Station 6. — August-Oct. 5m. Tows. 



IGimn. objective, I was unable to determine with accuracy species which are dis- 

 tinguished by minute details of structure, such as some of the Coscinodisci. In the 

 tables I have, therefore, grouped together the Thalassiosirae and the allied species 

 Coscmoswa poly chorda; and have included under their respective generic namee all 

 the Naviculae, AsterioTiellae, Surirellae, Campylodisci and Coscinodisci. 



After the material of each tow had been examined and all the species recorded, a 

 careful estimate of the numbers present was made in the following manner. The 

 volume of water, in which the organisms had been preserved, was increased to from 

 50 to 500ec. according as the amount of material was dight or abundant. In each 

 case the final volume was recorded. Counting was dene by means of a Rafter cell as 

 recommended by Moore (12). This consists of an ordinary glass microscope slide, 

 on which is fastened a rectangular rim of metal 5 cm. x 2 cm. and 1 mm. in depth. 

 This, therefore, when filled and covered with a slip contains 1 c.c. of liquid. To facili- 

 tate counting, a disc, on which was ruled a square, 1 mm. in area, was u.sed in the 

 eyeniece. The material was well stirred to insure a thorough mixing and to prevent 

 the accumulation of heavy forms at the bottom. While still in motion 1 c.c. was 

 quickly drawn off and placed in the cell. At least forty squares were counted in 

 €ach preparation and several slides were used from each collection. From the forty 

 or more squares counted the contents of each c.c. was reckoned; an average of the 

 contents of the several cells was then taken and this multiplied by the number pf c.c. 

 in the prepared material is an estimate of the number of individuals present. 



It will be noted that both in numbers and diversity of form the genus Chaetoceras 

 stands far in the lead. I-n September eleven species are recorded. The ranks are 

 then gradually thinned until during the winter only four speicies, Ch. dehile, diadema, 

 laciniostim and decipiens are found; and these are but scantily represented. The 

 addition of Ch. soriale in the spring adds greatly to the numbers; and from July 



