A. Smith Woodioard — Fish Fauna of Purhech Beds. 147 



The caudal fin is crushed in the type specimen and imperfect in the 

 other, so that the inequality or equality of the lobes cannot be 

 determined. 



Squmnation. — The whole of the trunk is covered with small, thin 

 scales, which have the appearance of overlapping ; but they are too 

 obscure for detailed description. A few large tubercles of ganoine are 

 distinctly observable on the flank of the abdominal region. 



Horizon and Locality. — Lower Purbeck : Teffont. 



Mesodon macropterus, Ag., var. parvus, no v. Plate YII. Fig 2. 



The type specimen of this variety is nearly complete, wanting only 

 the anterior portion of the head. It is shown of the natural size in 

 PI. VII. Fig. 2, and is sufficiently well preserved to enable the 

 principal characters of the trunk and fins to be ascertained. The 

 dorsal contour is much arched, though rounded, and the maximum 

 depth of the trunk in advance of the origin of the dorsal fin equals its 

 total length, measured from the hinder border of the operculum to the 

 extremity of the caudal fin. The head with opercular apparatus, 

 when complete, would probably occupy not less than one-quarter of 

 the total length. 



ITead and Opercular Apparatus. — Of the head, only fragments of 

 the hinder cranial roof-bones remain above, while an indecipherable 

 impression of the hyomandibular and its connections appears behind. 

 The preoperculum [p.op.)^ as usual is the largest ossification in the 

 opercular fold, this being triangular in shape, with the apex directed 

 upwards. The bone is twice as deep as its maximum breadth, and is 

 marked only by radiating lines. The operculum {op.) is relatively 

 small and narrow, tapering to a point below, and about two-thirds as 

 deep as the preoperculum. It seems to have been smooth externally, 

 being crossed only by one small oblique ridge. 



Axial Sheleton of Trunk. — The notochord must have been persistent 

 as indicated by the vacant space, and there are no ossifications in the 

 sheath. The bases of the neural and haemal arches, though robust, 

 are not much expanded. The neural and htemal spines are all fused 

 with their supporting arches and, except in the caudal pedicle, are 

 observed to be long and slender, bearing the ordinary median laminar 

 expansion in front ; the haemals at the base of the caudal fin, to the 

 number of about ten, are relatively robust and expanded distally for 

 the support of the rays. The total number of arches, excluding the 

 latter, is about thirty. 



Appendicular Skeleton. — Of the pectoral arch, the clavicle [cl.) only 

 is exposed in position behind the opercular bones. It is seen to be 

 expanded below and tapering above, with remains of the pectoral fin 

 posteriorly at a considerable distance above its inferior extremity. 

 The basal lobe of the pectoral fin exhibits seven of its supporting 

 cartilages radiating from its short attachment. Traces of very feebly- 

 developed pelvic fins occur at a point much nearer to the origin of the 

 anal fin than to the pectoral arch : they consist of delicate, distally 



1 The bone here named "preoperculum" has hitherto been named operculum, 

 while the "operculum" has always been described as supraclavicle. The writer 

 will shortly publish proof of the accui-acy of this amended interpretation. 



