Structure o/Coccosteus decipiens, Agassiz. 129 



the very same plate on the preceding page (p. 132, fig. 5, z z) 

 as forming a part of the ventral cuirass. 



The postero-lateral plate [p. I.) is a small one situated at 

 the posterior angulated margin of the lateral part of the cuirass 

 and articulates with the antero- lateral, the anterior dorso- 

 lateral, and the posterior dorso-lateral, its posterior margin 

 being free. This plate is not noticed by Pander or Huxley, 

 but it is lettered 2 by Hugh Miller (7, p. 133, fig. 6). 



The interlateral plate {%. I.) is one of great interest, as its 

 form and relations have not yet been properly recognized. 

 It consists of two parts, lateral and ventral, united at a con- 

 siderable angle to each other when uncompressed, which, 

 however, is very rarely the case. The lateral portion, seen 

 in fig. 1, forms a sort of fork, on which the short inferior 

 margin of the antero-lateral plate articulates, and thus is 

 formed that connexion between the dorso-lateral and ventral 

 portions of the cuirass which was unknown to Pander and 

 Huxley, and which, so far as I am aware, has not previously 

 been demonstrated. The lower limb of the fork forms a 

 conspicuous rounded lower margin, tuberculated like the other 

 plates, and bears a most suspicious resemblance to the part 

 represented by Prof. v. Koenen as a pectoral spine in G. 

 Bickensis (10, pi. ii. fig. 2). In C. decipiens it is, however, 

 very much shorter than the part alluded to in C. Bickensis ; 

 however, in C. minor it attains a very considerable propor- 

 tional length (13, pi. i. fig. 3, i. I.). The ventral portion (see 

 fig. 3), devoid of tubercular ornament, is elongated in shape, 

 and, passing inwards and slightly forwards to meet its fellow 

 of the opposite side, forms the anterior margin of the ventral 

 portion of the body-cuirass ; to it posteriorly are articulated 

 the anterior ventro-lateral and the anterior median plates. 

 This part of the bone was known to Pander, and is repre- 

 sented in two of his figures (6, pi. ii. fig. 2, and pi. v. fig. 1, 

 #), though in the text he compared it with the jugular plate 

 in Polypterus or Osteolepis. Huxley, on the other hand (8, 

 p. 35, fig. 21, a), considered the bone to be hyoidean in its 

 nature, as we have already noticed. 



Neither Pander nor Huxley seems to have recognized the 

 lateral portion of this bone, which serves to articulate the 

 dorso-lateral portion of the cuirass with the ventral ; indeed, 

 Huxley remarks (8, p. 32) that " the ventral shield appears 

 to me to have had no connexion with the dorsal." But of 

 the connexion of the two in the manner I have described 

 there cannot be the slightest doubt. See also my figure of 

 the parts in C. minor (13, pi. i. fig. 3). 



The plates forming the expanse of the ventral shield are 



