576 BRITISH PALAZOZOIC FOSSILS. [ Pisczs. 
without osseous adherence to the jaws, as in other fishes, nor placed in alveoli, succeeding each other from 
behind forwards by the rotation of their basal membrane; heart with a muscular bulbus arteriosus with many 
rows of valves ; intestine straight, with an internal spiral valve. 
Owing to the softness of the skeleton, it is only disjointed teeth, bony fin-defences, and rarely integument 
and vertebrie, which are usually found fossil. In some sharks the discoidal bodies of the vertebree are ossified, 
but the spinous processes are not. 
Fossil teeth of sharks were anciently called Glossopetrw ; those of Cestracions and Pycnodonts were called 
Bufonites. 
The Order is divided into two Tribes :—Ist, Holocephala ; 2nd, Plagiostoma. 
Examples of the Order are found recent in all latitudes, and in most formations from the Palzeozoic 
inclusive. 
2nd Tribe. PLAGIOSTOMA. 
This Tribe, containing the whole groups of Sharks and Rays, is easily distinguished from the Holocephala 
by the jaws, which bear the teeth, being loosely suspended, and moveable from the rest of the head. The 
bodies of the vertebrae usually form distinct discoid joints; the gills open to the external water in the adult by 
a row of apertures on each side. They are all exclusively marine. 
There are two families:—Ist, Squalidw or sharks ; 2nd, Rajidw or rays. 
Ist Family. SQUALID. 
Body elongate, fusiform, slightly compressed, slender, passing insensibly into the thick fleshy tail; gill- 
apertures lateral ; pectoral fins of small or moderate size. 
There are four Sub-families :—Ist, Squalinw ; 2nd, Hybodontine ; 3rd, Cestracionine ; 4th, Squatine. 
The two following fossils cannot be satisfactorily referred to their subfamilies. 
THELODUS PARVIDENS (Ay.) 
Ref —Ag. Sil. Syst. t. 4. f. 34, 35, 86; M°Coy, Quart. Geol. Journal, Vol. IX. p. 14. 
Dese.—V ery small quadrate granules, each averaging about one-third of a line in diameter ; height about 
equal to half the diameter; angles and lateral edges obtusely rounded ; upper surface flattened, highly polished 
(usually of a black colour); under surface slightly smaller than the upper one, not covered with ganoine, 
separated from the upper portion by a deep encircling lateral constriction ; in the middle of the under surface 
is a rounded pit, equalling one-third of the whole diameter, penetrating as a conical cavity nearly to the upper 
surface, being widest below. Vertical sections shew in the microscope very loose, tubular, slightly waved 
tissue, the lines arching upwards and outwards from the base, and from the sides of the conical internal 
cavity towards the part of the granules above the lateral constriction, becoming indistinct at a short distance 
from the polished surface. 
In a short memorandum written by me for Sir R. Murchison, (on the supposed Fish remains in his 
Silurian System) and published in the above volume of the Geological Journal, I made the following obser- 
vations, “All the specimens of the Downton Castle rock which I haye examined, impress me strongly 
with the conviction that the last name on the list, the Thelodus parvidens, should be considered not as that 
of a fish-tooth, but of granules of the skin or shagreen of the same fish, in all probability, of which fragments 
of the bony dorsal rays (Oxchus tenuistriatus) are so commonly intermingled in the same mass. M. Agassiz, 
judging only from the drawing (figs. 34, 35, 36,) supposed these magnified and isolated specimens to resemble 
teeth of the general character of Lepidotus; but one glance at the specimens would dissipate this notion, 
when we find that they are square and not rounded, that they are as small as grains of fine sand, and occur 
in such great abundance over large patches of rock as to resemble thick layers of sand. All these points 
