Homosteus, Asmuss, compared with Coccosteus. Agassiz. 55 



he assumed tliis tuberculated portion to he its " head," or 

 anterior extremity. The present specimen shows, however, 

 that this extremity was posterior. 



Tlie last and crowning point of interest in the specimen 

 represented in PI. III., Fig. 1, is tlie exhibition of the dorsal 

 cuirass iri situ. Five plates are here seen, one median and 

 two lateral, corresponding exactly to plates occupying a 

 similar position in Coccostetcs. 



The median dorsal plate (;m. d.) is so well known as Hugh 

 Miller's supposed "hyoid," that it requires no further de- 

 scription, beyond the remark that its resemblance to that of 

 Coccostr-'s, except in its short broad shape and the w\ant of 

 the posterior elongated peak, must be evident at the first 

 glance. It is here shown in position, its broad margin being 

 directed forwards, and lying parallel to the posterior margin 

 of the cranium, from wdiich it is here separated by a narrow 

 gap, its obtuse point being free and posterior. Its lateral 

 margins overlap on each side two other bones, of which the 

 anterior one (a. d. I.) was figured by Hugh Miller as "non- 

 descript latero-hyoidal plate," he being well aware of its 

 relation to the median bone, though, in accordance with his 

 theory of the latter, he reversed its position. Its relation to 

 the skull was, liowever, correctly represented by Pander 

 (" Placodermen," tab. 8, fig. 2a.). It consists of two parts, 

 one flattened above, and applied anteriorly to the outer part 

 of the posterior margin of the skull, by which as well as by 

 the median plate it is overlapped, and another, narrower, 

 forming a right angle to the flattened portion and running 

 forwards a little way along the posterior part of the outer 

 margin of the cranial buckler ; in the angle between the two 

 parts is a socket for articulation with a corresponding pro- 

 jection of the postero-external angle of the external occipital. 

 Naturally Hugh jMiller found himself at a loss to account 

 for the presence of this socket. Different as the two bones 

 are in shape, it is impossible not to recognise in this bone 

 the homologue of the anterior dorso-lateral in Coccosteus 

 (Fig. 3), though here the socket and peg have changed their 

 positions on the bones concerned. 



Behind this anterior dorso-lateral there exists in the speci- 



