1918] Barrows: Skeletal Variations in the Genus Peridinium 457 



size of these plates is in turn under control of whatever may be the 

 factor making for the orderly fragmentation of the shell and the 

 arrangement of its parts found in this group of organisms. Whether 

 this fundamental factor for plate growth proceeds by leaps and bounds 

 or not we cannot determine, because the very material of the shell in 

 which this factor is expressed may by some physical quality of its own 

 tend to retain its fragments in a given arrangement until a certain 

 accumulation of strain may become sufficient to cast it over into another 

 arrangement in somewhat the same way as the crystals of a kaleido- 

 scope retain a given pattern even though the barrel of the instrument 

 be moved through an appreciable arc of rotation before falling into 

 another pattern which in turn retains a certain degree of permanency. 



However, so far as the character of the arrangement of the plates 

 themselves and its expression in suture pattern are concerned, these 

 certainly behave as mutations. There is no suggestion that the pat- 

 terns are connected by a series of fluctuating or continuous variations, 

 but on the contrary, evidence is present that given patterns are not 

 so connected at all, and that a considerable gap of unknown relations 

 of the plates concerned remains between the alternate patterns in a 

 given variable region. This seems to be a perfectly clear and clean 

 cut result so far as can be judged from the mass evidence of a general 

 population. It is to be regretted that the cultural methods of the 

 protozoan geneticist have not been adapted as yet to this material. 



The independent behavior of each of these four variable areas in 

 certain cases, as well as the coupling of the dorsal and ventral areas in 

 pairs in certain other cases suggests that in these characters of vary- 

 ing plate patterns we may be dealing with unit characters in an 

 unusually simple and discreet form. 



Possible Influence op the Environment 



There is some evidence to suggest that conditions of the environ- 

 ment different from those of neritic or pelagic localities where given 

 species may be "at home" may be responsible for changes in plate 

 pattern as well as in other characters of dinoflagellates. Thus, for 

 example, such a form as P. granii is found in embayraents such as San 

 Francisco Bay and Union Bay, Alaska, and particularly in the Baltic 

 Sea where the salinity is very much lower than in the open ocean. An 

 even more striking instance comes from the specimens from San Fran- 

 cisco Bay which undoubtedly are related to P. oceanicum Van H. but 



