Oil tJic Structure 0/ Coccosteus decipiens, Arjassiz. 215 



though it is for the most part shut out from tliat by the 

 anterior dorso-lateral above and the interlateral below ; its 

 postero-superior margin, somewhat wavy or zigzagged, over- 

 laps the anterior dorso-lateral besides articulating with the 

 small postero-lateral. Tlie postero-inferior margin is free 

 and slopes obliquely downwards and forwards ; the short 

 anterior margin is fitted on to the interlateral. This antero- 

 lateral plate is the one lettered " c " by Huxley (8, p. 30) 

 and "3" by Hugh Miller (7, p. 133, fig. 6), though he has 

 represented 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, 



The interlateral plate (i. I.) is one of great interest, as its 

 form and relations have not yet been properly recognised. 

 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 connection 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 C. Bickensis (10, pi. ii., fig. 2). In C. decipiens it is, how- 

 ever, very much shorter than the part alluded to in C. 

 Bickensis ; however, in C. minor it attains a very considerable 

 proportional length (13, pi. iii., fig. 3, i. L). 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 



