Structure and Classification of the Asterolepidse. 39 



The median or "orbital" opening is in perfect specimens 

 of the head of Bothriolepis 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 duhiitm," 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 rifijht down throuorh 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 the Tunicata, as Prof. Cope has 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 we 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 pro- 

 cess described by Whiteaves be for the attachment of muscles 

 arising from the inner aspect of the shield, which, on con- 

 traction, would elevate the entire lid above the level of the 

 surrounding cranial plates, and enable the eyes to see out 

 from below its margins ? T 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 posi- 

 tion 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 maxillse." Except that their anterior margins 

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

 seen in Ptericlithys immediately in front of the semilunars, 

 and which Pander in Aster olepis has interpreted as " Unter- 



