Genus of Fossil Fishes from the Lias. 451 
in this fin as in the others, and along the front of the anterior 
ray there is a number of strong fulcral scales. The basal 
portion of each ray is grooved and smooth for about a quarter 
of an inch, beyond which the rays are articulated at short 
intervals and dichotomize freely. The caudal fin is large, 
bifurcated, heterocercal, and equally lobed. The vertebral 
column extends at least an inch beyond the termination of 
the body into the upper lobe of the tail, and the surface is 
covered with enamelled scales to an equal distance; from the 
osterior margin spring sixteen rays, which are articulated 
and freely dichotomize. ‘The ventral surface of the body 
from the anal fin to the root of the tail is protected by a series 
of large enamelled plates, which assume a fulcral aspect near 
the commencement of the lower lobe. The rays of the lower 
portion of the fin are stronger than those above, and are 1°6 
inch in length. They are articulated at short intervals, and 
each ray dichotomizes ; the external ones begin to divide 
at about 1 inch from the base, and only divide once, whilst 
those shorter and nearer the centre of the lobe divide into 
four, and, in a few cases, into six fine-jointed rays. The 
margin of the lobe is bordered by a row of small obliquely 
arranged slender fulcra. 
This ichthyolite, as already described, possesses many 
strongly marked characters, the most prominent being the 
extremely long jaws and wide gape, the sculptured surface- 
enamelled plates for the protection of the head, the anteriorly 
situated orbit, and the well-developed clavicles. The rhombic 
scales beautifully imbricating, with serrated posterior margins 
and smooth surface ; wide along the lateral line, very narrow 
ventrally. Pectoral and ventral fins paired, the former very 
large, composed of frequently- -dichotomizing closely-set rays. 
Single and large anal fin. Heterocercal tail with vertebral 
prolongation into the upper lobe ; the lower lobe, as well as 
the anal and ventral fins, having a series of fulcral scales 
along the anterior margin. ‘These characters indicate its 
relationsbip to the family Paleoniscide, as defined by Dr. 
Traquair*. Of the twenty-two genera included in this 
family by that author four have been obtained from the Lias, 
the remainder occurring in the older strata of the Permian 
and Carboniferous rocks. The four genera from the Lias are 
Centrolepis, Egerton; Oxygnathus, Egerton; Cosmolepis, 
Agassiz; and Thrissonotus, Agassiz. 
The genus Thrissonotus was instituted by Agassiz T for the 
accommodation of a fossil fish intermediate between Sauropsis 
* Paleontographical Society, vol. xxxi. (1877), 
+ Rech. sur les Poissons fossiles, vol. ii. pt. 2, p. 128. 
