1918] Barrows: Skeletal Variations in the Genus Peridinium 465 



nature of the superficial character of plate variation. There is no 

 evidence at all that the fundamental character for plate growth does 

 not proceed at an even pace. 



28. Plate formation in this family seems, therefore, to be the ex- 

 ternal expression of an internal force which is guided in its expression 

 by physical properties of the shell material in the formation, accord- 

 ing to a system consistent for the whole family, of plates limited in 

 size and guided also by geometrical, i.e., mechanical, limitations in 

 the arrangement of these plates in certain areas said to be more 

 "plastic" than other areas because manifesting the changes induced 

 by the internal force. 



29. There is also some suggestion that it may be the changes 

 themselves in the environment which indirectly induce certain of 

 these skeletal changes, since the asymmetrical patterns, especially of 

 the dorsal side, and the sudden transformation from one pattern to 

 another usually occur in species found under unusual conditions 

 of the environment, and in species which often show other features, 

 such as the abortion of antapical horn, indicating the shock of the 

 reaction of the organism to an unaccustomed environment. 



30. While for species in their natural habitat the skeletal patterns 

 seem to be on the whole a good and convenient species character as 

 well as one of sound phylogenetic basis, the occurrence of these occa- 

 sional sports indicates that it is not an infallible character, especially 

 in regions where a mixture of water from widely different sources 

 occurs or some other marked deviation from an accustomed environ- 

 ment. 



31. These conclusions in regard to the mutationary behavior of 

 these skeletal characters perhaps carry an added interest from the 

 occurrence of these phenomena in this group of marine Protozoa, liv- 

 ing under an environment very different in many respects from that 

 of land plants and animals which have been the usual subjects of vari- 

 ational studies, contributing perhaps support for the normal occur- 

 rence of a method of species formation by means of mutations. 



32. Though these observations may furnish some information upon 

 the behavior of the discreet elements of the dinoflagellate skeleton, 

 they do not contribute definite information upon the process of plate 

 growth in these organisms. This problem will require a different 

 method of attack, probably involving the continued observation of 

 single isolated specimens of dinoflagellates through considerable peri- 

 ods of time. 



