34 



Transactions of the Royal Canadian Institute 



Chelyosoma productum, Stimpson. 



We have had for study a large number of specimens of various sizes 

 from the coast of British Columbia. In this species the body is not 

 flattened or depressed but is elongated in a direction making a con- 

 siderable angle with the disk. As a result the latter is more symmetrical 

 than in the preceding spepies, there being less disparity between the 

 right and left sides and between the anterior and posterior ends. The 

 area of attachment to the substratum is small and the disk is relatively 

 smaller in comparison with the siphons and with the whole body. The 

 yellow layer is very thin and the test has only a slight yellowish tinge 

 even in large individuals. The extrasiphonal plates are less distinct 

 but more numerous than those of C. macleayanum. They also show 

 greater variation, but their symmetry and presumed mode of forma- 

 tion are the same. The atrial extrasiphonal series contains ordinarily 



Figure 2 — Scheme showing genesis of plates in C. Productum 



eight, that is, twjce the number of plates that is to be found in the same 

 series in C. macleayanum, and this condition is very evidently the result 

 of each plate dividing into two, as is most clearly seen in the case of the 

 central plate (PI. I, fig. 2). The oral extrasiphonal may show the same 

 condition, although frequently the division is carried a step further by 

 the original anterior plate being divided into four, making ten in all. 

 In respect to this tendency for the anterior end to take the lead in sub- 

 division of the plates, the two species agree. Text fig. 2 gives the pre- 

 sumed mode of origin. 



In the larger individuals of this species definite lines of growth are 

 to be seen on the plates (PI. I, fig. 2). Bancroft has noted aVid figured 

 these, but has attempted no explanation of their presence, though stating 

 that they are present only in the larger individuals. In our material 

 we have found by reference to these lines that there are very many in 



