128 , Dr. R. H. Traquair on the 



bones figured by Huxley as " the chief parts of the hyoidean 

 arch " are in reality the ventral rami of the dermal plates 

 which I have termed " interlateral." 



Body- Cuirass. — The front part of the body behind the head 

 is encircled by a girdle of osseous dermal plates, somewhat 

 comparable to a shoulder-girdle, expanded backwards dorsally 

 and ventrally, while at the lower part of the sides the cuirass 

 is so deeply cut in that the dorsal and ventral expansions 

 were long considered to have no connexion with each other. 

 Most of the osseous plates which form this cuirass are well 

 known from the writings of Pander, H. Miller, and Sir P. 

 Egerton, but nevertheless some correction is still necessary. 



The great median dorsal plate (fig. 2, m. d.) is of an elon- 

 gated pentagonal figure, its short base articulating with the 

 median and lateral occipitals, its acute apex and elongated sides 

 articulating with the two dorso-lateral plates, which it exten- 

 sively overlaps. Its under surface shows the well-known 

 median longitudinal ridge, ending behind in the " nail-head " 

 prominence, as in the corresponding plate in Homosteus. The 

 anterior dorso-lateral plate, the os articulare dor si of Pander, 

 (a. d. I.), is of a somewhat rectangular form when detached, 

 though in situ it appears irregularly trapezoidal owing to its 

 upper and lower margins being obliquely overlapped by the 

 median dorsal and by the antero-lateral respectively ; its 

 anterior margin shows a small articular process by which it 

 is joined to the external occipital. Immediately behind it is 

 placed the posterior dorso-lateral {p.d.L), or the os trian- 

 gulare of Pander, a triangular plate which also articulates 

 with the median dorsal above and the postero -lateral below, 

 while its oblique hinder border is free. 



The antero-lateral plate (a. I.), being the os marginale of 

 Pander, occupies a position below and in front at the nar- 

 rowest part of the lateral portion of the cuirass. It is pecu- 

 liarly trapezoidal in shape, or it might be described as trian- 

 gular, with the downward and forwardly directed apex 

 obliquely truncated. Its anterior border, gently convex in 

 the middle, forms part of the anterior margin of the cuirass, 

 though it is for the most part shut out from that by the ante- 

 rior dorso-lateral above and the interlateral below ; its postero- 

 superior margin, somewhat wavy or zigzagged, overlaps the 

 anterior dorso-lateral besides articulating with the small 

 postero-lateral. The 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 



