486 Dr. R. H. Traquair on the 



There is therefore abundant reason for going afresh into the 

 anatomy of the organisms discovered by Hugh Miller, Mal- 

 colmson, and Stables, and named by Agassiz Ptericldliys. 

 The special structure of the head and limbs was hitherto almost 

 unknown, and there is also room for rectification as regards 

 the body-carapace and tail. And this investigation is also of 

 great systematic importance as bearing on the question as to 

 whether Pander was right in maintaining the identity of 

 Agassiz's genus with Eichwald's Asterolepis ; for as ^^Aste- 

 rolfpis '^ has the priority, the only ground for maintaining 

 FtPrichthys^ were Pander right in his contention, would be 

 the inadequacy of Eichwald's original description of Astero- 

 lepis^ and then that name would have to be cancelled, as it 

 cannot legitimately be applied to the great Coccostean, 

 named Homosteus by Asmuss, and familiarly known to us as 

 Hugh Miller's ^'■Asterolepis of Stromness." With that 

 question is also bound up that of the distinction of Bothrio- 

 lepisj Eichwald, a genus also considered by Pander to be syno- 

 nymous with Asterolepis ; for although Lahusen (11) and 

 Trautschold (12) have given good reasons for retaining it as 

 valid, the latest writer on the subject, Whiteaves, treats the 

 question as one concerning which certainty has not yet been 

 attained (15). 



//ea^.— The cephalic shield oi Fterichthjs (PL XVII. fig. 1) 

 is of a semicircular or, rather, semielliptical shape, rounded in 

 front and truncated behind, where it joins the body-carapace. 

 In the centre it shows a transverse opening, distinctly represen- 

 ted in Hugh Miller's early drawing (o, pi. i. fig. 1), and w Inch, 

 though it was not mentioned by Agassiz, is nevertheless indi- 

 cated in his figures both of Ptericluhys tesludivarius (4, tab. iv. 

 fig. 2) and Famphractus hydrophilus {ib. tab. iv. fig. 6 and 

 tab. vi. fig. 2). This openings slightly contracted in the middle 

 and expanded on each side, I shall simply call ti)e median 

 opening J though it has usually been regarded as an " orbit," 

 and more recently Cope has put forward the view that it 

 represents tlie mouth in the Tunicata (17). It is entirely 

 filled up by a gniall plate or system ot plates rarely seen 

 in Ftericlithys, but, as we shall see further on, well dis- 

 played in many specimens of the allied genus Bothriolepis. 

 The nuchal region is occupied by a plate, the median occipital 

 [m. 0.) shaped somewhat like the conventional royal "crown," 

 but \\ ithout the pinnacle in the centre. Marginally it shows 

 six aspects or articulations — one posterior, straight, articula- 

 ting with the median dorsal plate of the cara))ace ; two lateral, 

 each of which passes first forwards and then obliquely for- 

 waids and outwards, articulating with the lateral occipital 



