ASTEROLEPIS MAXIMA. 79 



pre-median, a large four-sided plate, which behind forms the anterior boundary of 

 the orbit, while its anterior margin, showing a tolerably conspicuous median 

 indentation or re-entering angle, forms the middle part of the front of the shield. 



Two matters of detail, in which both the pre-median and the lateral plates par- 

 ticipate, must now be described. It will be noticed (PI. XVII, fig. 1) that the 

 anterior margin of the cranial shield formed by these two plates is distinctly 

 bevelled off by a narrow unsculptured band, which suggests the idea of some 

 additional plate having been once here articulated. So it is also in Asterolepis 

 omata, as described by Pander, who in his figures fits on to this place a narrow 

 transverse plate, his " os terminate" (' Placodermen,' pi. vi, fig. 1, No. 2). 

 This bone Pander admits that he had never procured perfect from Livonia, but 

 thought that he had found it in situ in Scottish examples of Pterichthys (ibid., pi. vi, 

 fig. 5). Nevertheless, on comparing the figure here quoted with numerous 

 specimens of Pterichthys, I am satisfied that the suture there indicated as marking 

 off the os terminate is only the transverse groove belonging to the lateral line 

 system which crosses the front of the band, so that I do not believe in the 

 existence of an " os terminate " in that genus (see above, p. 66). Indeed I can 

 see no absolute necessity in Asterolepis that an additional bone should here have 

 been articulated, as the same appearance might have been produced by the margin 

 of the shield in front of the sculptured surface having been covered with soft skin. 

 Then it must be observed that both in the lateral and pre-median the part of 

 the bone next the orbit is thickened, and produced downwards and inwards at the 

 sides, downwards and backwards in front, so as to furnish the orbit with anterior 

 and lateral walls, and to some extent also with a floor, thus converting it from a 

 mere opening into a cavity, — widely open, however, posteriorly. An idea of this 

 arrangement may be got by inspection of fig. 8 on PI. XVIII, representing an 

 impression of the outer surface of a cranial shield from which all the bone has 

 been removed ; and this consequently exhibits in the middle a prominent cast of 

 the orbital cavity, which is, however, slightly injured on the right side. This is 

 extremely interesting when compared with the condition of the orbit in Cephalaspis, 

 which appears to be floored below in a manner still more complete. 



Roofing over the middle of the orbital cavity is placed the median or pineal 

 plate (to.), a transversely oblong piece, with lateral margins concavely excavated to 

 complete each eye-circle. I have not seen in any specimen any trace of the 

 ocular plates, or of the additional median piece discovered by Whiteaves in 

 Bothriolepis. 



The last piece in connection with the head which has been found in Asterotepis 

 maxima is the extra-tateral or so-called opercular plate (e. I.). It is seen in the 

 specimen figured on PI. XV ; but, owing to its loose articulation to the side of the 

 shield, is invariably wanting in detached heads. It differs from that in Pterichthys 



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