38 Transactions of the Royal Canadian Institute 



The test in the neighbourhood of the apertures is divided more or 

 less regularly by grooves into series of plates in the following fashion, 

 A circular groove delimits each siphonal area. An elliptical groove 

 delimits the disk, of \yhich the apertures are the foci, and which is formed, 

 outside of the siphons, by the fusion of the extragiphonal areas of the two 

 apertures. Occasionally the t\v;o extrasiphon^al area's are imperfectly 

 divided by a groove half-way betw:een the siphons. Occasiona^lly the 

 extrasiphonal area is divided very imperfectly into an inner and an 

 outer series of plates, this affecting chiefly the atrial part and the left 

 side. 



The radial divisions from each aperture take place primarily in the 

 tw!o diagonal planes, giving four parts. Subsequent division takes place 

 in the median plane in the a,trial siphonal and the oral extrasiphonal area, 

 and in a transverse plane in the oral siphonal and the atrial extrasiphonal. 

 The extrasiphonal areate may show further division, in the median plane 

 in the case of the atrial area, and in the transverse plane in the case of 

 the oral. The oral may show additional division on either side anteriorly 

 in plan^ps midway between the median and the diagonal. While circular 

 division of the extrasiphonal plates takes place to a greater extent in the 

 more extensive atrial end and left side of the disk, radial division ad- 

 vances further in the less extensive oral end and right side. 



EXPLANATION OF PLATE. 



Plate I. 



Figure i. Chelyosoma macleayanum. Disk of individual from Arctic 



Alaska, in its fourth year. X 2. 



Figure 2. C. productum. Disk of individual from Ucluelet, B.C., in 



its second year. X 2. 



Figure 3. C. productum. Disk of individual from Ucluelet, B.C., in 



its seconid year, X 3. Second year's growth of plates 



predominantly on right side. 

 Figure 4. C. columhianum. Disk of individual from Departure Bay, 



B.C. X 3- 



