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CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3 d Ser. 



have examined the number is by no means constant. The 

 number of plates, exclusive of those composing the siphons, 

 varied from thirteen to twenty, the different numbers being 

 distributed as follows: 



No. of Plates 

 in Disk. 



No. of 

 Individuals. 



13 

 14 



2 



5 



15 

 16 



5 

 2 



17 

 18 



4 

 2 



19 

 20 



2 



1 



15-87 



= Average. 



But the variation is more than these figures indicate, for, in 

 the groups having the same number of plates in the disk, 

 the arrangement of the plates varied so much that in no 

 case more than three individuals were encountered whose 

 disks were alike in all respects. 



There are in all disks three well defined classes of 

 plates: 



(1) The si-phonal plates, which surround the orifices 

 and form the covering of the conical siphon. They are 

 quite constantly six to an orifice. In 24 individuals all had 

 six surrounding the branchial orifice, while one atrial siphon 

 had seven and another five plates. The branchial siphon 

 is almost invariably placed nearer the end of the disk than 

 the atrial, it is usually the longer and the area of its base of 

 attachment is less. Its apex, however, is usually less acute 

 and the plates do not fit so well as in the atrial siphon on 

 account of the greater development of the layer of test on 

 the inside of the siphon. 



(2) The central plates (c, fig. 3), which are placed 

 between the two siphons and are usually two in number. 

 A single one such as occurs in C. macleayanum has never 

 been met with; but twice three central plates were 

 encountered, a small median one having been interposed 

 between the other two. 



