242 = JS. C. Mansel-Pleydeli—On Histionotus, a Purbeck fish. 
and split. Each ventral fin, which is 20 mm. long, has only four 
rays, the first exceedingly stout and furnished with a series of large 
marginal fulcra; and all the rays are transversely articulated or 
jointed beyond the point of bifurcation. The anal fin has four rays 
also, and is the smallest of all; the length of the largest ray being 
15 mm. The caudal fin consists of eight rays in each lobe, the 
upper lobe appearing 40 mm. long, whereas the lower is only 
25mm. The margins of the upper ray of the larger lobe and of 
the lower ray of the smaller are furnished with a series of large 
fulcra which probably increased the locomotive power of the tail. 
The depth of the shoulder-girdle is 25 mm., and the breadth of 
the post-clavicular plates (p. cl.) 5mm. The opercular apparatus 
is well preserved, showing distinctly the operculum (op.), sub- 
operculum (s. op.), interoperculum (7. op.), and preoperculum 
(p. op.), besides a few branchiostegal rays (br.). These, like the 
post-clavicular plates of the shoulder-girdle, are all enamelled. 
The bones of the head are much crushed. The maxille, although 
perfect, are displaced; and a portion of the right premaxilla is 
preserved, showing seven teeth, with part of the eighth. These 
teeth are styliform, slender, straight, and smooth, and appear to 
decrease in size from front to back. The largest is 2 mm. in length. 
The vomerine bones are hidden, which unfortunately prevents our 
ascertaining whether or not the roof of the mouth is armed with 
teeth. The orbit is obliterated by the intrusion of the underlying 
skull-bones. 
The scales of the body are small as compared with those of 
Tepidotus, and are not so lustrous. Those of the flank measure 
6 mm. in depth and 2 mm. in breadth. They are rhomboidal in 
shape, finely pectinated on the posterior exposed margin, arranged 
obliquely in rows from the dorsal to the ventral region, twelve in a 
series. The lateral line from head to tail extends along the middle 
of the flank. Between the last ray of the dorsal fin and the base of 
the caudal, are three double-cone-shaped lustrous ridge-scales, which 
may have served to strengthen this attenuated part of the body, 
enabling it to resist the strain upon it when the caudal fin was in 
action. 
Length of body beh, (ame 1:80 m. 
fs tail, upper lobe Poi Leoni see aceerey mane s: USO) Wi, 
Be sie) MG WEEIODSWN ce). ssiu ee meme wescr pcset Tce aeNO man 
A head 500). Hae 0:46 m. 
Breadth of head 0:40 m. 
EXPLANATION OF PLATE VII. 
Histionotus angularis, Egerton; right lateral aspect of fish, nat. size- Middle 
Purbeck Beds, Swanage, Dorsetshire. dr. branchiostegal rays. 7%. op. interoper- 
culum. op. operculum. yp. cl. post-clavicular plates. yp. op. preoperculum,  s. op. 
suboperculum. : 
The specimen from which the above description is taken, and which is figured in 
the accompanying plate, was procured by me from a quarry at Herston, near 
Swanage, Middle Purbeck. It is now preserved in the Dorset County Museum, 
Dorchester. 
WHATCOMBE, BLANDFORD, DoRSETSHIRE. 
