6 GILBERT C. BOURNE. 



has described the right ctenidium as mono-pectinate, but, as 

 may be seen in fig. 7, it is really bi-pectinate ; the external 

 lamellas, however, are few in number, and in some specimens 

 are so feebly developed that they might easily be overlooked. 

 It is at first rather difficult to make out the details of the 

 structure of the ctenidia and to institute an exact comparison 

 between them and those of closely allied Aspidobi-anchs, but 

 a careful study of sections and whole preparations shows that 

 they are constructed on the familiar pattern. Each ctenidium 

 consists of an axis, the posterior part of which is fused to the 

 roof of the mantle-cavity and extends back in the angle of 

 that side of the mantle-cavity to which it belongs, lying just 

 above the columellar muscle. The anterior end of the axis 

 is free, and the large osphradial ganglion, as is always the 

 case in Aspidobrauchia, is situated at the point where the 

 axis becomes free from the mantle. This point, in Incisura, 

 corresponds with the anterior end of the columellar muscle. 

 In the case of the left ctenidium that part of the axis which 

 is fused to the mantle bears no filaments, but, as will be des- 

 cribed further on, this statement does not hold good for the 

 right ctenidium. Taking the left ctenidium for the purpose 

 of description : its free apex projects into the mantle- 

 cavity ill front of the columellar muscle as a thin, tri- 

 angular lamina, which, as already explained, is bent over 

 to the right, and also is twisted about its own axis from 

 right to left in such wise that the morphologically outer row 

 of filaments become posterior in position, the morphologically 

 inner row anterior. The efferent branchial vessel runs, as is 

 always the case, along the dorsal, here the posterior margin, 

 and the afferent vessel along the ventral, here the anterior 

 margfin of the axis. The inner and now anterior filaments 

 borne on the free portion of the axis are short and not more 

 than four or five in number, and are folded backwards over 

 the upper (morphologically ventral) side of the axis, appa- 

 rently as a result of the latter being twisted from right to 

 left in a narrow space. The morphologically dorsal edges of 

 the anterior filaments are consequently maintained in a dorsal 



