Structure and Classification of the Asterolcpidio 



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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 Jjothriolepis, Eichwald, a genus also con- 

 sidered by Pander to be synonymous 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). 



Head. — The cephalic shield of FtericJUhi/s (PL I., 

 Fig. 1) is of a semicircular or, rather, semi-elliptical shape, 

 rounded in front and truncated behind, where it joins the 

 body-carapace. In the centre it shows a transverse opening, 

 distinctly represented in Hugh Miller's early drawing 

 (3, pi. i., fig. 1), and which, though it was not mentioned 

 by Agassiz, is nevertheless indicated in his figures both of 

 Pterichthys teshcdinarius (4, tab. iv., fig. 2) and Fam- 

 phractus hydrophilics (ib., tab. iv., fig. 6, and tab. vi., fig. 2). 

 This opening, slightly contracted in the middle and expanded 

 on each side, I shall simply call the mediccn opening, though 

 it has usually been regarded as an " orbit," and more recently 

 Cope has put forward the view that it represents the mouth 

 in the Tunicata (17). It is entirely filled up by a small 

 plate or system of plates rarely seen in Pterichthys, but, as 

 we shall see further on, well displayed in many specimens 

 of the allied genus Bothriolepis. The nuchal region is 

 occupied by a plate, the median occipital (in. o.), shaped 

 somewhat like the conventional royal " crown," but without 

 the pinnacle in the centre. Marginally it shows six aspects 

 or articulations — one posterior, straight, articulating with 

 the median dorsal plate of the carapace ; two lateral, each of 

 which passes first forwards and then obliquely forwards and 

 outwards, articulating with the lateral occipital (/. o.) ; tw^o 

 antero-lateral, passing at an angle forwards and inwards, 

 articulating with the lateral plate on each side; and one 

 anterior, retracted in the centre, so as to form a ^vide 

 re-entering angle in which the correspondingly angulated 

 posterior aspect of the postmedian plate {p. m.) is received. 



