498 Dr. R. H. Traquair on the 



The median or " orbital " opening is in perfect specimens 

 of the head of Botliriole'pis filled up by a system of plates, 

 being the " Decke ^' already noted by Trautschold. Whit- 

 eaves describes the arrangement as consisting of four elements, 

 one central, like Pander's " os dvhium^'' one anterior, and 

 two lateral, of a rounded form, stating besides that the ante- 

 rior one shows a remarkable slender process passing from the 

 middle of its anterior surface right down through the head. 

 I have not seen these plates in B, canadensis^ but the " lid " 

 is well shown in two specimens of B. hydropliilus in the 

 Edinburgh Museum, in which the rounded lateral parts are 

 seen to be very convex above. I cannot in these specimens 

 trace any separation into distinct plates ; but this may be 

 due to mode of preservation. 



Whether this median opening represents morphologically 

 the mouth of theTunicata, as Prof. Copehas suggested (17,18), 

 or not, the lateral convexities of the lid distinctly indicate that 

 it covered a paired organ or pair of organs ; and what paired 

 organs could wx more readily suppose to occupy this position 

 than the eyes? But of what use could the eyes be if covered 

 above by an opaque bony roof? Here I would venture a 

 suggestion. May not the slender descending process described 

 by Whiteaves be for the attachment of muscles arising from 

 the inner aspect of the shield, which, on contraction, would 

 elevate the entire lid above the level of the surrounding 

 cranial plates, and enable the eyes to see out from below its 

 margins ? 1 do not put forward this theory with any notion of 

 infallibility, but it does seem to me more consistent with the 

 actual arrangement of the parts than that which supposes 

 the median opening to be a mouth, the position of which was, 

 I think, more probably on the under surface of the front of 

 the head. 



On the under surface of the head Whiteaves figures two 

 plates (13, pi. vii. fig. 1, no. 15), of which he says that they 

 " no doubt correspond to the plates which Pander calls the 

 lower maxillje." Except that their anterior margins come 

 too far forwards, these plates do remind us of the pair seen in 

 PierichtJiys immediately in front of the semilunars, and which 

 Pander in Asterok^ns has interpreted as " Unterkiefer." 

 Is it not possible that the exceeding closeness of their ante- 

 rior margins to the edge of the cranial shield in Whiteaves's 

 figure may be due to a slight forward displacement, 

 such as often occurs in Pterichtliys to a much greater 

 extent ? In a specimen of B. canadensis in the Edinburgh 

 Museum remains of these plates occur, which evidently are so 

 displaced, as they are shoved forwards quite over the edge 

 of the cranial shield. 



