Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/867

 The posterior extremity, however, is a little enlarged, and appears to have been the articular surface.

The enlarged part of the anal fin (b) is placed near to the transverse centre of the body. It is about half an inch long, and is triangular, with the extremity pointed, the posterior margin being slightly convex, the anterior straight or a little concave. The base extends nearly as far forwards as the posterior ventral plates, being prolonged in this direction by seven or eight pointed scale-like rays or plates, which diminish in length forwards; and it is continued backwards to the root of the tail as a low fringe (b'), about 3/16 inch wide, supported by rather distant delicate rays, but whether jointed or not could not be determined. The anterior elongated portion is composed of between twenty and thirty simple, jointed rays, the joints being rather long, with the articular extremities slightly enlarged.

The dorsal fin (a) has a very extraordinary development ; its anterior elongated portion is estimated to be as long as the whole fish minus the tail, and rises up from the dorsal ridge a little in advance of the centre, consequently just in front of the position of the anal fin, in the form of a scimitar slightly bent forwards. It seems to have been in the larger specimens upwards of 4-1/4 inches in length, and is extremely narrow, being only 2/10 inch wide an inch from the base, gradually tapers to a fine point above, and as gradually widens downwards to within 3/8 inch of the base, which suddenly expands, stretching forwards almost as far as the occiput, and backwards to the root of the tail, as a narrow fringe, similar in height and appearance to that on the opposite or ventral margin. The anterior prolongation (a") is composed of a double lateral series of about twelve low, stout, pointed, scale-like rays or processes, the fin-fulcra, inclining and increasing in length backwards. The first of the series is extremely short, just appearing above the dorsal ridge ; the last is about 4/10 inch in length ; and the whole, leaning in the direction of the fin, one supporting the other, form, as it were, a sort of buttress to the base of the enormously elongated fin. Only a few of the central rays reach to the top of this elongated portion of the fin ; both in front and behind they die out gradually. At the base, just above the anterior fin-fulcra, there are twelve or thirteen rays ; halfway up they are reduced in number to seven or eight ; and within half an inch of the top, judging by the upper portion of a fin in our possession, there are only four or five. All the rays are divided into numerous and unusually elongated joints (fig. 4), which at the points of articulation are slightly enlarged. The joints are longest in the centre or anterior rays, some of them measuring 1/10 inch in length.

The tail is distinctly heterocercal, and is deeply and regularly forked, the upper lobe being only a little longer than the under. The former is 1-1/4 inch in length; and from tip to tip of the lobes the tail is 1-1/2 inch in width. The anterior margin of the upper lobe is defended by a closely arranged series of stout, sharply pointed, enamelled, scale-like processes or fin-fulcra (fig. 1, f), which imbricate and diminish in size backwards. Behind or below these there is a double row of rhombiform scales (e), likewise covered with enamel