AN UNDESCRIBED A CRANIATE. 225 



anteriorly as well as posteriorly. According to Lankester (23) this 

 is the case in amphioxus if we regard the right half of the pre-oral 

 hood as a continuation of the right " epipleuron." But in the 

 Bahama amphioxus both metapleura seem to be continuous with 

 the side of the hood, these with the edges of the anterior part of the 

 pre-oral chamber and these in turn with the single median fin (Fig. 6). 



In Fig. 13, which is a section across the base of the hood, the 

 connective tissue mass joining the shrunken-in hood membrane to 

 the lateral muscle mass of each side is a direct continuation of 

 and is some sort a representative of the metapleuron of each side. 

 Again in the section Fig. 1 2, anterior to the free edge of the hood 

 the same connective-tissue masses now project freely and form edges 

 to the pre-oral chamber. They are much like the fins, or meta- 

 plura, posterior to the atriopore (Fig. 15). 



Even if we may thus regard the metapleura as continuous with 

 the sides of the pre-oral hood and the anterior edges of the pre-oral 

 chamber, we are puzzled by the fact that they meet at the anterior 

 end of the pre-oral chamber and became continuous with the 

 median fin (Figs. 6, 20). At the same time the right predomi- 

 nates here also and it is chiefly the right side of the pre-oral 

 chamber which is fringed by a continuation of the median fin. As 

 the median fin is thus posteriorly continuous with a single paired 

 structure, the right metapleuron, and anteriorly continues with what 

 appears to be two paired structures, both metapleura, we see no sure 

 ground for inferring its origin, either from the loss of one, or from 

 the fusion of both of a primitive pair of organs. Yet the indications 

 are that the median fin is not so much single by fusion as by loss 

 of its fellow. 



As illustrating the ease with which paired lateral structures may 

 pass to a median ventral position and vice versa, this case is of 

 interest with reference to the origin of appendages of fishes from 

 lateral fins and their connection with a median fin. 



The disappearing metapleura on the sides of the pre-oral chamber 

 are intimately associated with the pre-oral hood. This structure 

 is much more extensive than in the common Amphioxus, forming a 

 deep shovel-shaped or inflated membrane suspended from the pos- 

 terior and lateral margins of the pre-oral depression; when ex- 

 panded it appears as in Fig. 6. 



