214 Proceedings of the Royal Physical Society. 



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 

 liead is encircled by a girdle of osseous dermal plates, some- 

 what 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 connection 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 (Pig. 2, m. d.) is of an 

 elongated pentagonal figure, its short base articulating with 

 the median and lateral occipitals, its acute apex and elon- 

 gated sides articulating w^ith the two dorso-lateral plates, 

 which it extensively overlaps. Its under surface shows the 

 well-known median longitudinal ridge, ending behind in the 

 "nail-head" prominence, as in the corresponding plate in 

 Homosteiis. The anterior dorso-lateral opiate {a. d. I.), the os 

 articulare dorsi of Pander, is of a somewhat rectangular form 

 when detached, though in situ it appears irregularly trape- 

 zoidal 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 

 {jy. d. I.), or the os triangulare 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 r)iarginale of 

 Pander, occupies a position below and in front at the 

 narrowest part of the lateral portion of the cuirass. It is 

 peculiarly trapezoidal in shape, or it might be described as 

 triangular, 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, 



