60 OLD RED SANDSTONE FISHES. 



purpose it is as well to take Pterichthys, as being the genus best known in its 

 entirety, if not in every matter of detail. 



The nomenclature of the parts here employed is essentially the same as that 

 used in my essay " On the Structure and Classification of the Asterolepidge," 

 published in 1888. 1 



The body or carapace consists of osseous plates closely fitted together, closed 

 above, below, and at the sides, but open in front for the head, and behind for the 

 tail. The head is represented almost entirely by a dorsal shield, formed also of 

 plates united by suture. The two pectoral limbs or " arms " consist also of 

 plates similarly united, .and are hollow tubes like the legs of an insect so far as 

 their remains preserved in the stone are concerned. 



The head-shield is semicircular, or rather semi-elliptical in shape, rounded in 

 front and truncated behind, where it joins the body-carapace. In the centre it 

 shows a transverse opening, the median opening or orbit, slightly contracted in 

 the middle and expanded at each of its rounded sides. This opening is in perfect 

 specimens filled up by at least three other plates, which, being loose, are usually 

 lost. Of these one is in the centre, quadrate in shape, but with concave outer 

 margins, and may be called the median or pineal plate (m.), as it shows on the 

 internal aspect a shallow rounded pit, pointed out by Mr. Smith Woodward as 

 probably the impression of the pineal body. This is by its outer concave margins 

 in contact on each side with a rounded convex ocular plate (o.), indicating certainly 

 the position of the eye, but whether or not due to an ossification in the sclerotic is 

 doubtful. The nuchal region is occupied by a large plate, the median occipital 

 (m. occ), shaped somewhat like the conventional royal "crown," but without the 

 pinnacle in the centre. In front of this and immediately behind the median 

 opening is a smaller plate, the post-median {pt. m.) ; while between the anterior 

 margin of that opening and the front of the cranial shield is one of larger size 

 and somewhat quadrate shape, the pre-median (p. m.). Two large paired pieces, 

 the lateral plates (/.), one on each side, bound the median opening laterally, and 

 also extend to the front of the shield, behind which, and forming part of the hinder 

 margin of the buckler external to the median occipital, are two other paired plates, 

 the lateral occipital (I. occ.) and the angular (ag.). 



The upper and lateral aspect of the cranial shield is now completed by a plate 

 on each side, which is only loosely articulated in Asterolepis and Pterichthys, though 

 firmly sutured in Bothriolepis. This is the extra-lateral (e. I.) or opercular plate, 

 as it has also been called by some writers. 



Pander has represented in Asterolepis, somewhat hypothetically, a narrow 

 plate, the so-called os terminate, forming the very anterior margin of the shield in 



1 ' Ann. and Mag. Nat. Hist.' (6), vol. ii, pp. 485 — 503 ; see also Smith Woodward's ' Catalogue 

 of the Fossil Fishes in the British Museum,' pt. 2, pp. 208—212. 



