36 Transactions of the Royal Canadian Institute 



The yellow layer of the test is very thick, giving the animal, except 

 in specimens tha^t appear to be young, a decidedly yellow colour. The 

 extrasiphonaj platen tenifi to be somewhat irregular and their number is 

 large and quite variable. To explain the condition let us take the 

 ordinary condition in C. macleayanum as a starting-point. This explan- 

 ation of the origin of the plates is illustrated in text figure 3. We have 

 seen that in the oral siphon the two lateral plates of the four primary 

 ones are divided by a transverse cleft, making six in all. In the extra- 

 siphona.1 oiral airea, on the cointrary, the primary four plates are increased 

 in number, not by a transverse division, but by a longitudinal or median 

 division, which affects only the anterior plate, making a total of five. 

 In C. columbianum this median division ajffects also the posterior member 

 of the oral extrasiphonal series, that is the central plate, giving a minimum 

 number of six plates, (as seen in Plate I, fig. 4), which agree in position 



Figure 3 — Scheme showing genesis of plates in C. Columbianum 



with the six plates of the atrial siphon. Usually there is further division, 

 which curiously enough is more apt to afifect the two anterior plates than 

 the individual lateral plates of either side. Of the anterior plates the 

 right is more frequently double than is the left. In the extreme condition 

 there are ten plates in this series, derived from the original four plates as 

 follows, anterior, 4; right lateral, 2; left lateral, 2; posterior (central), 2; 

 which is the condition in C. productum. 



In the atrial siphon the original four plates have become six by a 

 median division of the anterior and posterior plates. In the extra- 

 siphonal series the original four are increased by a transverse division of 

 the lateral plates, which may affect only the right lateral. If both are 

 divided, we would have six, but the anterior member of the series (the 

 central plate) is common to both oral and atrial areas and has already 

 been divided in its relation to the oral area, so that we have seven as 

 the usual number of extrasiphonal atrial plates (PI. I, fig. 4), that is one 

 less than in C. productum. 



