DEPARTMIiNT OF" MARINE BIOLOGY. 137 



hypothetical explanation is offered, that limitation in growth is caused by the 

 natural limit of asexual reproduction of a sexually produced polyp. To state 

 this hypothesis in other words : a sexually produced polyp may bud or divide ; 

 each polyp thus asexually produced can give rise to only a limited number of 

 asexual generations, and if sexual reproduction does not intervene the species 

 will become extinct. 



Busmilia knorri. — In 1910 observations or measurements were again made 

 on two colonies, the records for which began in 1908, and one, the record for 

 which began in 1909, all three colonies growing attached to the piers of the 

 Fort Jeft'erson dock. Although the data are meager, they indicate rapid 

 growth, the increase in the three dimensions varying from 33 per cent to 100 

 per cent, and the increase in the number of calices from 100 per cent to 200 

 per cent per annum. 



During 1910 the number of records was increased to 14, 10 of them being 

 without measurements and 4 with measurements. Each of two of three speci- 

 mens first measured in 1910 consisted of a single undivided calice. The next 

 season's field observations may furnish sufficient data to determine the time 

 required for growth from the initial calice to an average-sized mature 

 specimen, 



Dichocoonia stokesi. — A single colony of this species, living in the Fort 

 Jefferson moat, is under observation. It is attached to the vertical face of 

 the outer wall, near the southeast exit; it is permanently submerged and 

 through the nearby entrance receives a supply of pure sea-water. The speci- 

 men is large : Length in 1909, 363 mm. ; in 1910, 368 mm. ; an increase in one 

 year of 5 mm., or 1.38 per cent. Breadth in 1909, 297 mm. ; in 1910, 315 mm. ; 

 an increase of 18 mm. or 6 per cent. The measurement of the thickness in 

 1909 is not reliable. The growth of this colony was slight, and as it is already 

 large perhaps it has almost reached its growth-limit. 



Oculina sp. probably diffusa. — In 1908 records were begim for three colo- 

 nies attached to the piers of the Fort Jefferson dock; in 1909 records were 

 begun for one additional colony attached to a pier and for two growing in 

 the moat. Three colonies, attached one above another to the same pier of 

 the dock, show the following percentage increase in height : the uppermost, 

 175 per cent ; the median, 75 per cent ; the lowest, 33.33 per cent. The upper- 

 most colony has increased in height more than five times as rapidly as the 

 lowest. As the uppermost has the strongest light, this may be the accelerating 

 factor. Another colony on a pier showed an increase in height during the 

 year from 75 to 128 mm., a percentage increase of 70 per cent. Two colonies 

 in the moat showed respectively an increase in height of 41 to 62 mm., or yy 

 per cent, and 35 to 60 mm., or 86 per cent. The increase of the number of 

 branches in these two colonies was about 100 per cent. 



In 1910 a young Oculina was discovered beside the two others in the moat, 

 and as it was not seen in 1909 probably represents a year's growth. Its 

 dimensions are as follows : Diameter of basal expansion in horizontal plane, 

 32 mm. ; in vertical plane, 28.5 mm. ; one branch projects horizontally 22 mm. 

 from the basal expansion. A large colony on a dock pier is 350 mm. in diam- 

 eter and 100 mm. tall. The annual growth-rate in height is from 15 to 35 

 mm., with about 25 mm. as an average. Therefore, a colony may grow to a 

 height of 100 mm. in four years under moderately favorable conditions ; the 

 range in the attainment of such a size would probably be from three to seven 

 years. 



In 1910 the number of growth records on this species was increased from 

 6 to II, 



