ASTEROLEPID^E. 67 



front of the pre- median and lateral pieces, but of whose existence in Pterichthys at 

 least I have never been able to see any evidence. I shall return to this question 

 under Asterolepis. 



On the lower aspect of the head and close behind the anterior margin of the shield 

 are two transversely oblong plates (m. x.), right and left, the position of which 

 was first determined by Whiteaves in Bothriolepis? an observation corroborated 

 by Smith Woodward. 2 These plates must have been situated in front of the mouth, 

 and may therefore be lettered, at least conventionally, as maxillae. There can be no 

 doubt that they were similarly placed in Pterichthys, though I formerly assigned 

 to them, as mental plates, 3 a position behind the mouth, and in Asterolepis they 

 were designated maxillae inferiores by Pander. Close to the postero-external 

 angle of each of these plates there is a rounded notch, considered by Smith 

 Woodward in Bothriolepis as possibly indicating a nasal opening. 4 



The body-carapace is box-like, nearly quite flat below and vaulted above. It is 

 composed of thirteen plates, of which three are median and ten paired, and these 

 are united to each other by overlapping sutures, a marginal band along the internal 

 surface of the overlapping plate being excavated to fit on to a correspondingly 

 excavated band along the margin of the outer surface of the plate overlapped. 



On the upper surface we see the anterior and posterior median dorsal plates 

 (a. m. d. and p. m. d.) succeeding each other in the middle line and on each side ; 

 passing down also on the lateral wall are the anterior and posterior dorso-laterals 

 (a. d. I. and p. d. I). On the under surface, and also taking part in the formation 

 of the lateral wall, are two pairs of plates, the anterior and posterior ventro -lateral 

 (a. v. I. and p. v. /.), of which the anterior requires special attention, as to it the 

 pectoral limb is articulated. Near the anterior extremity of this plate, on the outer 

 aspect and close above the angle which separated the lateral from the ventral surface, 

 is a shallow excavation, from the bottom of which rises a peculiar process (b. p., 

 woodcuts, Figs. 38 and 39, p. 78) resembling a thick-walled cup or helmet, whose 

 hollowed-out " mouth " points outwards, and also somewhat backwards and down- 

 wards, the cup itself being fixed by a stout ridge which traverses the containing 

 hollow from behind forwards and also slightly downwards. This may be called the 

 brachial process (helmet-process of Pander), as it is grasped by the two articular 

 plates of the upper arm, and thus forms the brachial joint. Immediately behind 

 this brachial process is a small oval aperture, the brachial foramen (jr., woodcut, 

 Fig. 39), perforating the brachial fossa from the interior, and which no doubt 

 served to convey to the arm the blood-vessels and nerves required for its supply. 



1 ' Trans. Boy. Soc. Canada,' vol. iv, sect, iv, 1887, pp. 103, 104. 



2 ' G-eol. Mag.,' (3), vol. ix, 1892, p. 484. 3 Loc. cit., p. 488. 



4 This notch is somewhat differently placed in Bothriolepis, being fair on the outer margin of the 

 plate instead of at its postero-external angle. 



