98 



OLD RED SANDSTONE FISHES. 



plate (a. m. d.) is pretty well shown as regards its external outline, but the posterior 

 part of the upper aspect of the carapace has splintered off, uncovering the upper 

 surfaces of the posterior ventro-lateral plates (p. v. I.) below. 



On the same plate, Figs. 2 and 3, we have the dorsal and ventral surfaces of 

 the unique specimen in the Hugh Miller Collection in the Royal Scottish Museum, 1 

 Edinburgh, already described and figured by Agassiz as the type of his 

 PL testudinariiis, and referred to at page 90 of this work as showing both surfaces. 

 The dorsal surface erroneously interpreted by Agassiz as the ventral is seen in 

 Fig. 2, and shows well the general form of the head, the position of the orbit, and 

 the form and relations of the median occipital (m. occ.) and post-median {pt. m.) 



Fig. 53. 



Diagrammatic outline of the contour of the ventral surface of the body-carapace and left pectoral 

 appendage in the ordinary form of Pterichthys Milleri. 



plates. On the body-carapace the bone is nearly altogether lost, but a pretty 

 good impression of the under surfaces of the plates is retained, in which all their 

 outlines may be discerned, the posterior median dorsal (p. m. d.) being, however, 

 somewhat deficient behind. On the under surface of the specimen (Fig. 3) the 

 bone, though mostly preserved, is divided by such a number of reticulating cracks 

 as to assume also a tessellated appearance. Yet the outlines of all the elements of 

 this surface, except the maxillary, are distinctly traceable, though the right 

 posterior ventro-lateral (p. v. I.) is injured at its hinder extremity. But the semi-limars 

 (.<?. I.) behind the mouth are well defined, as well as the median ventral (m. v.), which 

 appears small because its margins are largely overlapped by the lateral plates and 

 only the sculptured area is exposed. 



In PI. XX, fig. 1, the configuration of the ventral aspect is well shown, 

 though the body-plates are seen from their smooth internal or upper surfaces. 

 1 Formerly the " Museum of Scieuce and Art." 



