424 University of California PuUications in Zoology [Vol. 18 



to occur coincidently in two dorsal regions, while the two ventral 

 regions of change seem often to vary independently of each other. The 

 changes occurring at the two pairs of variable regions have been dia- 

 grammed in figs. 1 to 8. Fig. 3 shows an asymmetrical ventral plate 

 pattern, which is formed by the non-coincident variation of the pair 

 of ventral regions of variation in the reverse order from that of fig. 2. 



It is to be noted- that figs. 1 and 4a show the only possible com- 

 binations of the plates of this part of the epitheca if the two sym- 

 metrically opposite regions of variation behave coincidently. But a 

 stage represented by fig. 2 must have occurred in the formation of the 

 stage given in fig. 4a, unless that of fig. 4a arose by direct symmetrical 

 progression from the stage given in fig. 1, which, as has been ex- 

 plained, is presumed to represent the primitive condition for these 

 plates in this genus. This stage shown in fig. 2, however, has not been 

 found by the writer in anj^ specimen, and if we may except the figure 

 of Claparede and Lachman (1859, pi. 13, fig. 26), which may have 

 been drawn from the inside of the shell, or from the dorsal side, in- 

 stead of in correct orientation, this pattern has not been figured in 

 literature. 



The variations of the dorsal region involve the possibilities for 

 the articulation of plates about the anterior right and left corners of 

 the mid-dorsal precingular plate, with readjustment on the part of 

 the adjacent accessory plates (la, 2a, and 3a) and also of the abutting 

 precingular plates (3" and 5"). 



It will be noted that in each of the regions which undergo varia- 

 tion there are only four plates the rearrangement of which is involved 

 in changing from one pattern to the other; that in this quartette of 

 plates it is the diagonally opposite pairs of plates which are either 

 separated or brought into juxtaposition with each other; that only 

 two patterns are possible for each quartette in a given variable 

 region; that one of these patterns must therefore be regarded as the 

 alternate of the other ; that the patterns change by the obliteration of 

 the suture between two plates about to be separated in forming the 

 new pattern, and by the formation of a new suture at right angles 

 to the one just obliterated through the newly affected contact of the 

 alternate pair of plates. 



With one exception all possible combinations for these plates in 

 this pair of variable regions are known, and are presented in figs. 1 

 to 8. It may be repeated, then, that in the most fully developed 

 presumably the most characteristic, and certainly the most abundantly 



