442 University of California Puhlications in Zoology [Vol. 18 



cess of evolution may have passed, by showing many different sizes 

 of the accessory plates, seem now to be lacking, possibly having per- 

 ished as unsuited for some reason to the conditions under which they 

 would have been compelled to live. These conditions, in fact, them- 

 selves changing, may have been the stimulating cause for this series 

 of steps in the development of this genus. Though now missing from 

 our peridinian fauna as at present known, it is not impossible, how- 

 ever, that under certain conditions which might upset the equilibrium 

 of species which find their way into bays, brackish areas, etc., rever- 

 sions to or repetitions of the early stages of this development may occa- 

 sionally be found. 



Be the origin of the accessory plates as it may, we find the 

 greater portion of the genus characterized by having three accessory 

 plates, with at least four patterns for the arrangement of the constant 

 number of plates in this general mid-dorsal region of the epitheca. 

 two sjTiimetrical patterns and two asymmetrical. 



Origin of the Principal Symmetrical Dorsal Plate Patterns. — In 

 this portion of the genus we may suppose, then, that at a stage 

 shortly after the development of the third discreet accessory plate this 

 plate became of approximately the same size as the two lateral acces- 

 sory plates. At this stage the plate pattern would be that of fig. 5, 

 the middle accessory plate having met the anterior edge of the mid- 

 dorsal precingular plate, 4", and flattened out against a considerable 

 portion of the anterior margin of this plate. 



By a continuation of skeletal growth in this region of the shell, 

 which seems to be one of plasticity because of the recent introduction 

 in this region of these additional plates, plate 2a, the middle accessory- 

 plate, may be presumed to have continued to increase in size. At any 

 rate, this plate is found to var^' in proportionate size as compared 

 with adjacent plates. 



When of its smallest size the middle accessory plate presents to- 

 gether with the adjacent plates the pattern of fig. 5, which is charac- 

 teristic of such forms as P. di^ergens Ehrbg., P. crassipes Kofoid, and 

 P. oceaniciim Van H. When of a larger size it seems to have spread 

 still farther along the anterior edge of the mid-dorsal precingular 

 plate, 4", and to have intruded itself between the former articulations 

 of plates la and 4" on the left and plates 3a and 4" on the right sep- 

 arating these plates entirely and reaching to plate 3'' on the left and 

 to plate 5" on the right, presenting the plate pattern of fig. 6, one 

 which is characteristic, for example, of P. conicum (Gran) 0. and S., 



